A "rational" number has nothing to do with sensibility. It means a number that can be expressed as a ratio.
Regarding A and B, since I can see at a glance the answer is D I'm not going to waste my time with them.
The square root of 3 cannot be expressed as a ratio, so multiplying it by 0.5 won't change that.
Assuming only one answer is correct, that answer is D. 1/3 x 0.5 = 0.5/3 = 1/6