# of French speakers in 2003 = 4800 * 0.21 = 1008
# of French speakers in 2004 = 6000 * 0.21 = 1260
--> Difference = 1260 - 1008 = 252

The answer is E.

Alternatively:
Since the percentage of French speakers has remained constant, we can just calculate 21% of the difference in population.
Difference in population = 6000 - 4800 = 1200
Increase in French speakers = 1200 * 0.21 = 252