Answer: Option b; son las seis menos cuarto.
Explanation: We want to find a way to say the time 5:45.
let's analyze the options one by one:
a) "son las cinco menos cuarto"
means something like "there are 15 minutes to five o'clock" or 4:45
b) "son las seis menos cuarto"
means something like "there are 15 minutes to six o'clock" or 5:45
So this is the real answer.
"5:45 son las seis menos cuarto"