need some help

B 4 days.

Explanation: from the chart it's apparent that 75% of the isotope 200 has transformed in the isotope 196 (it couldn't be the opposite, that is 25% of the 196 into the 200, because it can't gain mass). Half-life means that after that time half of the initial isotope decays, so an half-life of 4 days means that 50% of the initial quantity of 200 has transfomed into 196. So in 4 days we are left with 50% of the 200. After other 4 days this amount will be halfed again, so it will remain 25%. In conclusion: after 8 days will remain 25% of the initial isotope 200 and the rest 75% has become the isotope 196, as results from the chart.