Step 1: Meaning of the sentence.
The correct preposition with "happy" is "with," which shows satisfaction regarding someone or something.
Step 2: Apply grammar rule.
We say "happy with someone" to express satisfaction.
Step 3: Eliminate incorrect options.
(A) by, (B) at, and (D) for do not fit the idiomatic usage.
The manager asked the team to complete the project _______ the end of the week.