The correct option is
A will help
Answer : A
Explanation:
Quotation marks are used to mention the exact words spoken during a conversation.
So basically it can be understood as the written form of verbal communication.
And to talk about things that have not happened yet, we use the future tense.
Specifically we use the simple future tense to talk about things that will start and end in future.
Looking at the alternatives:
--will help : "will" is used in sentences of future tense, and with it we use
the base / root form of the verb. Now here it is important to know that
we use "will" when we are making a rapid/on-spot decision or a prediction.
--is
going to help : future tense; "going to" indicates the intention /
plan
of the speaker; and with that we use the base form of the verb. Please
note that we use "going to" when talking about a decision that was made
prior to the conversation.
--helped : past tense
--helps : 3rd person present of the verb "help"
Here, in the second sentence the other person asks the first person to ask Joe to help and then he presents his opinion that he would help him. Also it was his on-spot decision / opinion and it was not pre-planned.
Thus grammatically most suitable alternative here is "will help".
Therefore the correct complete sentence is:
"My car isn't working."
"Ask Joe to look at it. He WILL HELP you."