Answer:
d) 90 degree
Step-by-step explanation:
Note the below properties of a rhombus;
*Diagonals bisect the angles of a rhombus
*In a rhombus, diagonals bisect each other at right angles
From the second statement, we can deduce that the below angles are right angles, that is, they are equal to 90 degrees;

As seen above, angle BFC is a right angle, that is, it is 90 degrees