Answer:
The correct answer is The man who sat beside me at lunch, speaks French.
Explanation:
Relative clauses are sentences with relative pronouns (who, which, whose, that) used to define and identify a thing or person that has already been mentioned before and we want to add more information.
To understand whether it is necessary to add a comma or not, you must ask yourself if the information you are adding is extra or necessary.
In this case, "The man who sat beside me at lunch, speaks French." The fact that he speaks French is not relevant with respect to the person to whom "who" is referring.
A necessary information would be “sat beside me at lunch”, since without that information it could be talking about any man.
