a.
Never true. The angles of an equilateral triangle are 60° each, so an equilateral triangle can never be a right triangle which has one angle 90°.
b.
Possible. An isosceles right triangle has angles 45°, 45°, 90°.
c.
Possible. The angles of an equilateral triangle are 60° each, so it's always an acute triangle.
d.
Possible. For example a triangle with angles 70°, 70°, 40° - the angles are less than 90°, so they are all acute, and there are two angles with the same measure, so it's an isosceles triangle.
The answer is A.
