Explanation:
this is just a matter of a very simple definition :
an acute triangle has only angles that are smaller than 90°.
an obtuse triangle has one angle that is larger than 90°.
a right or right-angled triangle has one angle with exactly 90°.
keep in mind, the sum of all angles in a triangle is always 180°.
so, there cannot be 2 angles with 90° or more than 90°.
therefore,
A : acute
B : obtuse
C and D : right