JEE Main 2025 Apr 4 Shift 2, Physics Q1: Radioactivity

The chain is $P\to Q\to R$, with R stable.

P: decays by a first-order law, so its mass falls monotonically from its initial value toward zero (exponential decay).

Q: is produced from P at a rate ${\lambda }_P[P]$ and removed at a rate ${\lambda }_Q[Q]$. Initially $[Q]=0$, so production exceeds removal and $[Q]$ rises. Eventually production from the dwindling P cannot keep up with removal, so $[Q]$ peaks and then falls back to zero. The Q curve is bell-shaped.

R: is stable and only accumulates, so its mass rises monotonically from zero and saturates at the original total mass.

Only option (2) shows all three features simultaneously: P decaying, Q peaking, R saturating.