1 pipe can completely fill the tank in 6 hours 

therefore, $\frac{6 \mathrm{parts}}{6 \mathrm{hours}}=\frac{1 \mathrm{part}}{1 \mathrm{hour}}$

After first pipe fills 3 parts (half tank), 3 more pipes are added.

Now altogether four pipes are filling the remaining 3 parts (half tank)

3 parts of the tank can be filled in 3 hours (180 min) by each of the pipes

1 pipe -- 3 parts -- 180 min

4 pipes -- 3 parts -- X

$$\begin{gathered} =\frac{1\times 3\times 180}{4\times 3\times \left(\mathrm{x}\right)} \\ =\frac{180}{4\mathrm{x}} \\ \therefore \mathrm{x}=\frac{180}{4}=45 \min \end{gathered}$$

So it takes 45 minutes to fill the remaining tank.

Hence total time taken by all the four pipes to fill the tank is,

3 hours by first pipe + 45 min by all four pipes together

Total 3 hours 45 minutes.

Hence option D is correct.