A

Step 1: Convert the mass of the bullet from grams to kilograms.
The mass of the bullet, m = 20 g = 0.02 kg.

Step 2: Calculate the speed of the bullet.
The distance (d) between the two points = 30 m.
The time interval (t) = 4 s.
Speed (v) can be calculated using the formula:
$$ v = \frac{d}{t} $$
Substituting the values:
$$ v = \frac{30 \text{ m}}{4 \text{ s}} = 7.5 \text{ m/s} $$

Step 3: Calculate the kinetic energy (KE) of the bullet.
The formula for kinetic energy is:
$$ KE = \frac{1}{2} mv^2 $$
Substituting the values of m and v:
$$ KE = \frac{1}{2} \times 0.02 \text{ kg} \times (7.5 \text{ m/s})^2 $$
$$ = \frac{1}{2} \times 0.02 \times 56.25 $$
$$ = 0.01 \times 56.25 = 0.5625 \text{ J} $$

Therefore, the kinetic energy of the bullet is 0.5625 J.