30 may also be the top angle (30, 75, 75).

Then these two angles are not similar. )

(b)- It is possible that 45 is the base angle (45, 45, 90).

It is also possible that 45 is the vertex angle (45, 67.5, 67.5).

Then these two angles are not similar. )

(c)- When 60 is the base angle (60, 60, 60).

When 60 is the vertex angle (60, 60, 60)

An isosceles triangle with an angle of 60 is an equilateral triangle, and all equilateral triangles are similar.

(D)—— When 80 is the base angle (80, 80, 20).

When 80 is the vertex angle (80, 50, 50)

Then these two angles are not similar. )

So choose (c)