1. Decay scheme

A radioactive substance $P$ decays to $Q$, and then $Q$ decays to stable $R$:

$P \rightarrow Q \rightarrow R$

Here:

• $P$ is radioactive, so its mass decreases continuously with time.
• $Q$ is produced from $P$ and destroyed by its own decay into $R$.
• $R$ is stable, so its mass keeps increasing with time.

2. Behavior of mass of $P$

Since $P$ decays radioactively, its mass falls exponentially:

$m_P(t) = m_{P0} e^{-\lambda_1 t}$

So the graph of $P$ must:

• start from maximum value at $t=0$
• decrease monotonically
• approach zero asymptotically

3. Behavior of mass of $Q$

At $t=0$, usually there is no $Q$ initially, so:

$m_Q(0)=0$

Now:

• $Q$ is being formed from decay of $P$
• simultaneously, $Q$ itself is decaying into $R$

Hence, initially $Q$ increases, because production dominates. Later, after some time, decay of $Q$ becomes significant and its mass decreases.

Therefore, the graph of $Q$ must:

• start from zero
• first increase
• reach a maximum
• then decrease back toward zero

So $Q$ must have a hump-shaped curve.

4. Behavior of mass of $R$

$R$ is non-radioactive, so it accumulates continuously from decay of $Q$. Thus:

• it starts from zero
• increases monotonically with time
• finally approaches a constant value equal to the total initial mass (ignoring mass defect issues at this level)

So the graph of $R$ must be a rising curve that saturates.

5. Identify the correct figure

The correct figure must show:

• $P$: decreasing continuously
• $Q$: first increasing, then decreasing
• $R$: continuously increasing

Among the given options, this corresponds to Option B.

6. Final answer

$\boxed{B}$

7. Comparison with stored answer

Stored correct answer = B

My derived answer = B

So they agree.