A radioactive material P first decays into Q and then Q decays to non-radioactive material R. Which of the following figure represents time dependent mass of P, Q and R?

Substance P (Parent): Initially starts at a maximum value ($$M_0$$) and undergoes continuous, pure exponential decay. Its mass decreases monotonically toward zero: $$M_P(t) = M_0 e^{-\lambda_1 t}$$
Substance Q (Intermediate): Initially starts at zero. As P decays, Q is continuously produced while simultaneously decaying into R.

At early times, its production rate from P is higher than its own decay rate, causing its mass to rise to a maximum peak. Eventually, as the supply of P depletes, its decay rate dominates, causing its mass to fall back asymptotically toward zero.

Substance R (Stable End-Product): Initially starts at zero. Because it is non-radioactive (stable), it never decays. It accumulates continuously over time, flattening out asymptotically into a maximum steady plateau ($$M_0$$) as all of P and Q are completely spent.

Option (B) shows it correctly.