Answer:
(a) increase by
times
Step-by-step explanation:
Natural frequency of a wave in a string is given by:

where, L is the length of the string, T is the tension in the string and
is the linear density of the string.
Considering the length and linear density of the string are constant, if the tension in a string is doubled, the natural frequency of the string would:


Thus, the natural frequency of the string would increase by
times.