Short answer A
This one is exactly the same (with number changes) as the last one. You cannot use t which is in time, to mix with pure numbers which in this case is grams. That means that both C and D are incorrect.
Now as with the last one, are you going to raise e to a minus number or a plus number? Remember that if e is raised to a plus number, the sample in this case will increase. You are watching a radioactive decay. The number has to be smaller. So B is eliminated. There is only one answer left and that's A. It should be correct.
A <<<<< answer