Answer:
The correct answer is 2. I often think about taking a kickboxing class.
Step-by-step explanation:
In this sentence, the modifier often is clearly linked to think. In the other two sentences, the modifier is not clear. It could go two ways. In I told Lucy when the test was over we should celebrate, it could mean that the person told Lucy they should celebrate after their test, or when the person saw Lucy after the test, she said they should celebrate.