To find the percentage increase in the population:

1. Calculate the increase in population:
\[ \text{Increase} = 6000 - 4000 = 2000 \]

2. Calculate the percentage increase based on the original population:
\[ \text{Percentage Increase} = \left(\frac{\text{Increase}}{\text{Original Population}}\right) \times 100 \]
\[ \text{Percentage Increase} = \left(\frac{2000}{4000}\right) \times 100 = 0.5 \times 100 = 50\% \]

So, the percentage increase is:

A. 50%