Solution

Answer is A.

When tuning a violin, a string is tightened which means that the length of the string is shortened.
When the length of a string is changed, it will vibrate with a different frequency. Shorter strings have higher frequency and therefore higher pitch. When a musician presses his finger on a string, he shortens its length. The more fingers he adds to the string, the shorter he makes it, and the higher the pitch will be. That is, higher frequency.
Hence, the statement is true.