Which term means starting an activity or process?

• A. Hit the books
• B. Get the ball rolling
• C. Once in a blue moon
• D. Blow one's own horn

Answer: (B)

The expression that means starting an activity or process is the one that uses the idea of beginning motion, like initiating a project or discussion. Get the ball rolling captures this sense perfectly: it pictorially shows starting something and gaining momentum. In use, you might say, “Let’s get the ball rolling on this plan,” to signal you’re ready to begin and push things forward.

Other phrases here describe different ideas. Hit the books means to study, once in a blue moon means something happens rarely, and blow one’s own horn means to boast about oneself. So the phrase about starting something is the best fit for the concept of initiating an activity.