Correct Answer: twelve months

Solution : The correct option is - twelve months

If the radioactive substance's half-life is six months, it means that in six months, 50% of the material will have decayed. Only 50% of the material is left when it decays. After another six months, the residual substance will decompose to half its original volume. Therefore, three-quarters of the material, or 75%, will degrade within a year .