nice replies.
and yes the correct answer is B
here is a justification.
as your brake pads wear out, the brake oil level drops in container because brake caliper requires more travel from actual position to braking postion.
this is not un-usual or is considered a fault or problem, it comes in usual car maintanance,
good brake pads are suppose to wear out quick to stop the car and bad quality brake pads will wear out slow.
if you choose answer A and fill up brake oil , then what happens is when you change worn out pads with new, the new pads will make brake oil overflow which you had added later.
so always chk pads or have them checked if you cant locate them