Explanation

(b)
Quantity of $H_{2}O_{2} = 15\ mL$ and volume of $H_{2}O_{2} = 20$
We know that 20 volume of $H_{2}O_{2}$ means 1 L of this solution will give 20 L of oxygen at NTP.
Since, oxygen liberated from 1000mL (1L) of
$H_{2}O_{2} = 20\ L,$ therefore, oxygen liberated from 15mL of $H_{2}O_{2}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = \frac{20}{1000} \times 15 = 0.3\ L = 300\ mL.$