Person who gives written testimony for use in law court

Answer: Option A

One word substitute is Deponent.

Deponent: a person who makes a deposition or affidavit under oath.
Appellant: a person who applies to a higher court for a reversal of the decision of a lower court.
Witness: a person who sees an event, typically a crime or accident, take place.
Litigant: a person involved in a lawsuit.