Answer:
d. 45°
Explanation:
You want to know which from your list is a standard elbow angle.
Elbow
An elbow is a pipe fitting that is used to change the direction of piping. Elbows can be used together to produce an offset and/or a change in the plane of the piping.
Standard elbow angles commonly are 90°, 45°, and, less commonly, 22.5°. The greater the angle, and/or the smaller the bend radius, the more resistance to flow it has.
The best choice of answer here is 45°.
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Additional comment
The typical elbow has the same kind of threads on both sides. A "street elbow" has male threads on one side and female threads on the other side.
Elbow joints are not necessarily threaded. They may also be soldered, welded, or compression joints.