The correct answer is: A. A straight glacis is provided.
A glacis is a sloping embankment that is used to protect a structure from erosion or damage. In Montague type falls, a straight glacis is provided to protect the structure from the impact of the falling water. The glacis is typically made of concrete or stone and is sloped at an angle of approximately 30 degrees.
Option B is incorrect because a circular glacis would not be as effective at protecting the structure from the impact of the falling water. Option C is incorrect because a parabolic glacis would be more difficult to construct and would not provide as much protection as a straight glacis. Option D is incorrect because a glacis is always provided in Montague type falls.