Power = (amount of energy moved or work done) / (the time it takes)
100 joules of energy used in 0.5 seconds = (100/0.5) = 200 watts
500 joules of energy used in 0.5 seconds = (500/0.5) = 1,000 watts
100 joules of energy used in 0.25 seconds = (100/0.25) = 400 watts
500 joules of energy used in 0.25 seconds = (500/0.25) = 2,000 watts
The last choice describes the greatest power.