The correct answer is: A. Less than d.
A conical flare is a cone-shaped section that is attached to the top of a cylindrical stack. The diameter of the base of the conical flare is always less than the diameter of the cylindrical part. This is because the cone shape tapers down, so the diameter of the base is always smaller than the diameter of the top.
Option B is incorrect because the diameter of the base of the conical flare is always less than the diameter of the cylindrical part. Option C is incorrect because the diameter of the base of the conical flare is always less than the diameter of the cylindrical part. Option D is incorrect because the diameter of the base of the conical flare is always less than the diameter of the cylindrical part.