A 40 W bulb is used for 15 minutes. Calculate the heat generated.

The power of the bulb ($P$) is $40 \, \text{W}$.
The time ($t$) the bulb is used is $900 \, \text{s}$.
The voltage ($V = 120 \, \text{V}$) is given but not needed for this calculation since power is already provided.

The heat generated ($Q$) by an electrical appliance is the product of its power and the time it is used.

Comparing this result with the given options:
(A) $1.8 \times 10^5 \, \text{J}$
(B) $3.6 \times 10^4 \, \text{J}$
(C) $4.8 \times 10^4 \, \text{J}$
(D) $6.2 \times 10^4 \, \text{J}$

The calculated heat generated matches option (B).

The final answer is $\boxed{\text{B) } 3.6 \times 10^4 \, \text{J}}$.