If a coin is flipped 20 times and a head comes up 7 times, what is the relative frequency of a head coming up?
a. 0.45
b. 0.35
c. 0.30
d. 0.40

Relative frequency is the absolute frequency (i.e. the number of times of the wanted result) divided by the total  number of events.

In this case you have to calculate: the number of heads that came up divided by the number of times the coin was flipped.

Number of times the coin was flipped: 20
Numbef of times head came up: 7

Relative frequency: 7 / 20 = 0.35, which is the option B.