The correct answer is:

A. took on

So the complete sentence would be:

"That the boss took on another three clients enabled us to improve our income."

Explanation:
- "took on" means to accept or undertake additional responsibilities or tasks, which fits the context of acquiring more clients.
- "caught up" means to reach the same level or standard as someone else, which doesn't fit the context.
- "caught on" means to become popular or understood, which also doesn't fit the context.
- "took up" means to start doing something regularly, which is not the best fit for the context of acquiring clients.