According to Reference Table N, which radioactive isotope will retain only 1/8 its original radioactive atoms after approximately 43 days?

A. gold-198

B. iodine-131

C. phosphorus-32

D. radon-222

Reference table N you presumably have.

The half life of a substance is the time it takes to half in mass, and so 1/8 means 3 times the half life, which in this case is going to be approximately 14 days.

Lucky for you google has the half life of those isotopes, and having looked them up I can tell you that the answer is C - Phosphorous-32, as it's half life is 14.29 days, which when multiplied by 3 gives approximately 43.