The correct answer is B: A single null. The feature of a cardioid pattern antenna that makes it useful for direction-finding antennas is a single null. The single, well-defined null clearly indicates the direction of the signal. Cardioid patterns have one main lobe (forward direction) and one null (reverse direction). This single null is perfect for direction-finding because when you rotate the antenna and find the null, you know exactly which direction the signal is coming from - there's no ambiguity. This is much better than a bidirectional pattern (like a loop without termination) which has two nulls, creating ambiguity. The single null of a cardioid pattern makes it ideal for DF work - you can precisely determine signal direction by finding the null.