Explanation:

- Let's assume pipe A fills the tank in 'x' hours.

- Therefore, pipe B will take 'x + 6' hours to fill the tank alone.

- Together, they fill the tank in 4 hours:

$$

\frac{1}{x} + \frac{1}{x+6} = \frac{1}{4}

$$

- Solving for 'x':

$$

\frac{x+6 + x}{x(x+6)} = \frac{1}{4}

$$

$$

\frac{2x + 6}{x^2 + 6x} = \frac{1}{4}

$$

$$

4(2x + 6) = x^2 + 6x

$$

$$

8x + 24 = x^2 + 6x

$$

Rearrange and solve:

$$

x^2 - 2x - 24 = 0

$$

$$

(x-6)(x+4) = 0

$$

- So, x = 6 or x = -4 (not possible as time cannot be negative).

- Option 2: 6 hrs

The correct answer is that pipe A takes 6 hours to fill the tank alone.