These three resistors are all in parallel, so the formula to use is:
1/Req = 1/R1 + 1/R2 + 1/R3
where Req is the equivalent resistance of the parallel resistors
So from the given,
1/Req = 1/4 + 1/4 + 1/2
1/Req = 1
Req = 1 ohm
So the equivalent resistance of the circuit is 1 ohm, which is choice A.