The angle between skew back of a flat arch and the horizontal, is kept approximately equal to A. 0° B. 30° C. 60° D. 90°

The correct answer is $\boxed{\text{B}}$, 30°.

A skew back is a backfill or retaining wall that is built at an angle to the horizontal. It is used to support the weight of the earth and prevent it from sliding down. The angle between the skew back and the horizontal is typically kept at 30°. This is because this angle provides the most stability for the structure.

If the angle is too small, the skew back will not be able to support the weight of the earth and it will collapse. If the angle is too large, the skew back will be unstable and it will also collapse.

Therefore, the angle between the skew back and the horizontal is kept at 30° to provide the most stability for the structure.

Here are some additional details about each option:

• Option A: 0°. This is not a possible angle for a skew back. If the angle were 0°, the skew back would be vertical and it would not be able to support the weight of the earth.
• Option B: 30°. This is the correct answer. The angle between the skew back and the horizontal is typically kept at 30°.
• Option C: 60°. This is not a possible angle for a skew back. If the angle were 60°, the skew back would be unstable and it would collapse.
• Option D: 90°. This is not a possible angle for a skew back. If the angle were 90°, the skew back would be horizontal and it would not be able to support the weight of the earth.