Answer: (a) KP
Step-by-step explanation: We know that, if two planes intersect with each other, then their intersection is a line. Since HKP and RKP are two planes, and the points (vertices K and P) are common to both of them, so the intersection of these two planes is a line made by joining these two points K and P. So, KP is a line common to both the planes HKP and RKP.
Thus, the correct option is (a) KP.