Answer: B. ST.
Step-by-step explanation: The intersection of two planes, if exists, is a straight line. Since the planes STXW and SVUT have S and T as common points and we can always draw a straight line given any two points in a plane, so ST is the line joining the points S and T, and hence, it is the intersection of the two given planes.
Thus, the correct option is B. ST.
