Question 4 of 20 : select the best answer for the question. 4.   which of the following would lower the pitch of a string instrument's sound?           a. shortening the effective string length.   b. increasing the thickness of the string used.   c. adding tension within the string.   d. using less force to vibrate the string

The correct option is B.
There are four basic factors which affect the properties of stringed instruments, these are frequency, diameter [thickness], tension and density.
Diameter refers to the thickness of the string. Thick strings with large diameter vibrate slower and thus they have lower frequency and lower pitch compared with thin strings.