Answer:
a. plan
Explanation: It wasn't to long ago that I worked on this.
Use this as an example to understand it better->
They PLAN to endorse
vs
They PLANS to endorse
Can you see the error there, and which one sounds and looks correct when shortened like that?
I'm sure you chose plan for it. That's an easy way to see whether or not the answer is correct. If you need more help understanding these types of questions, I can, and will definitely help you out! You can add me on a social if you don't want to use up points for it. :)