I'm thinking Lack of Motivation. I'm going to double check though for you.
If the half-life is 5 minutes, then 30 minutes is equivalent to 6 half-lives. So, after six successive half-lives, the 400 mg of the radioisotope would have a mass of 400(0.5)⁶ = 6.25 mg.
Thus, A would be the correct answer choice here.
It’s a homework routine. hope this wasn’t too late :)