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.
