One computation in 5*10^-7 seconds (0.0000005 seconds) means that the computer can do:
1/(5 * 10 ^ -7) = 1/0.0000005 = 2 000 000 operations per second
4 Hours = 4 x 60 x 60 = 14 400 seconds.
So in 4 hours, the computer can do:
14400 x 2 000 000
= 28 800 000 000 computations
= 2.88 x 10^10
So the answer is (c), above.