The correct machining sequence is drill, ream, tap. This sequence is commonly used in metalworking processes. First, a hole is drilled in the material using a drill bit. Then, the hole is further refined and smoothed using a reamer. Finally, a tap is used to create internal threads in the hole. This sequence ensures that the hole is properly prepared for tapping and helps to achieve accurate and high-quality results in the machining process.

Form is a control that refers to the overall shape and configuration of a feature. It ensures that the feature is in the correct shape and does not deviate from its intended form. In the context of the given question, straightness is a form control that ensures that a feature, such as a line or surface, is perfectly straight without any deviations or bends.

A French curve is a drawing tool that is used to draw smooth curves that are not circular. It is particularly useful for creating precise and accurate freehand curves in drafting and design work. The French curve has a variety of curved edges of different shapes and sizes, allowing for the creation of a wide range of curves. This makes it a versatile instrument for drawing curves that are not easily achievable with other tools such as a compass or protractor.

Ebony does not belong to a T-square because it is not a part of the tool. A T-square typically consists of a blade, stock, and a working edge. Ebony, on the other hand, is a type of wood known for its dark color and density. It is not a component or characteristic of a T-square.

The tolerance zone shape for circularity is represented by two concentric circles. This means that the acceptable range for the circularity of a feature is defined by two circles, one inside the other. The measured feature must fall within this tolerance zone to be considered within the acceptable range for circularity.

A divider is a tool that can be used to transfer dimensions when there is a mistake in repetition. It allows for precise measurements and can be adjusted to the desired length. By using the divider, one can easily transfer dimensions accurately without any errors or repetitions.

The answer is "orientation" because datum reference is used to establish a point, line, or plane from which the location or orientation of other features on a part are measured. Orientation refers to the angular position or direction of a feature relative to the datum reference. Therefore, orientation control must contain datum reference to accurately define the angular position or direction of a feature. The other controls listed (form, straightness, and profile) do not necessarily require datum reference.

French curves are generally used to draw smooth, curved lines, not horizontal lines. Protractors are used to measure and draw angles, not horizontal lines. Set squares are used to draw straight lines at specific angles, not necessarily horizontal lines. T-squares are primarily used to draw horizontal and vertical lines, making them the most suitable tool for drawing horizontal lines. Therefore, the correct answer is T-square.

The initial construction of lines is drawn using an H pencil. The "H" stands for hardness, and the higher the number, the harder the pencil lead. H pencils are known for their ability to create light and fine lines, making them suitable for initial sketches and construction work.

The profile control is used to specify the allowable variation in the shape or contour of a feature. It defines a tolerance zone around the true profile of the feature. The datum reference is used to establish a reference point or axis from which the profile is measured. Therefore, the profile control may or may not use a datum reference, depending on the specific requirements of the feature being inspected.