Question

The _____ is responsible for understanding the advertiser's marketing and promotional needs and interpreting them to agency personnel.

Answer 1

Your answer is account executive 

Answer 2

Hey there!

Your answer is an account executive.

An account executive, like you said in the question, is responsible for understanding the advertiser's marketing and promotional needs and interpreting them to agency personnel.
As stated in the name of the term, they are there for a company or advertiser to meet their needs financially.

Hope this helps!