What percentage of a given mass of a radioactive substance will be left undecayed after five half-life periods?

3.125%

The correct answer is Option (2) → 3.125%

Fraction remaining after n half-lives: $N = N_0 \left(\frac{1}{2}\right)^n$

Given: $n = 5$

$N = N_0 \left(\frac{1}{2}\right)^5 = N_0 \frac{1}{32}$

Percentage remaining: $\frac{N}{N_0} \times 100 = \frac{1}{32} \times 100 \approx 3.125\%$

Percentage left undecayed = 3.125%