If a plane slices a rectangular prism through three points that are equidistant from one of its vertices, which of the following best describes the cross-section of the prism

An equilateral triangle would be formed.

First, imagine that you have a cube and are looking at one of the corners (vertices). Measure down the three sides from the corner the same distance.

Next, imagine that you take a knife and cut a straight plane through those points. The result of your cross section would be an equilateral triangle.

The answer of the above is