Leila's interference in her sister's wedding plans has led to nothing but trouble.
Explanation:
We are supposed to substitute the part "has only caused trouble" for "has led ______ trouble". As was shown in the question, we are supposed to use the word "nothing" as well as two other words that keep the same meaning as "has only caused trouble" conveys. Therefore, the best option is "has led to nothing but trouble". It expresses the same idea and uses "nothing" and two other words - "to" and "but".
Let me start with why it isn't C. C is true but it's not more important than what A says. A talks about how Martin and Kennedy agree with their words which is what the question asks.
