We can first find the other two angles in the triangle. We are given that one external angle is 90 degrees. Then, its linear pair inside the triangle is 180 - 90 = 90 degrees.
Similarly, the other external angle is 125 degrees. Its supplement inside the triangle is 180 - 125 = 55 degrees.
The total degrees of a triangle should be 180. We have two of the three angles: 55 + 90 = 145 degrees.
To find p which is the third angle, we just do 180 - 145 = 35 degrees.
So, the answer is (C).