Pipe A can fill a tank three times faster than pipe B. If both pipes A and B are running together, they can fill the tank in 12 minutes, then the time taken by pipe B alone to fill the tank is:

48 minutes

The correct answer is Option (4) → 48 minutes

Let the rate of pipe B be $r$ tanks/minute. Then pipe A's rate is $3r$ tanks/minute.

Together, rate of A + B = $3r + r = 4r$

Time to fill the tank together: 12 minutes

So, $(\text{Rate}) \times (\text{Time}) = 1$ tank

$4r \cdot 12 = 1 \Rightarrow 48r = 1 \Rightarrow r = \frac{1}{48}$

Time taken by pipe B alone:

$\text{Time} = \frac{1}{r} = 48$ minutes