The correct answer is A: It modifies the pattern of a DF antenna to provide a null in only one direction. The function of a sense antenna is to modify the pattern of a direction-finding (DF) antenna to provide a null in only one direction. It resolves the bidirectional ambiguity of loop antennas. A sense antenna is a small omnidirectional antenna (often a short vertical) used with a loop antenna for direction-finding. The loop has bidirectional nulls (two nulls 180 degrees apart). By combining the loop's output with the sense antenna's output in the right phase, you create a cardioid pattern with a single null. This eliminates the ambiguity - you now know which direction the signal is coming from. The sense antenna 'senses' the direction, resolving the bidirectional ambiguity. This is a common technique for improving DF accuracy with loop antennas.