A unknown piece of metal is placed into a container of acid. Hydrogen gas was produced as a result. The unknown metal could have been ? Ti is the answer, but I don't understand why.

Answer 1

any metal more active than hydrogen. See the activity series:

http://www.grandinetti.org/resources/Teaching/Chem121/Lectures/ActivitySeries/ActivitySeries.png

anything higher on the chart will displace anything lower.