Answer: Phosporic acid or hidrochloric acid.
Explanation: There are not options, but the two acids that i know are used to remove rust are:
Phosphoric acid "converts" the iron oxide (rust) into ferric phosphate, which you can scrape after and in this way remove the rust.
Hydrochloric acid is used in the steel industry to pickle" steel by removing rust or scale, but the hydrochloric acid can affect other metals, so you may need to be careful when utilizing it.