option A. I guess you only took the factors of 9500 which is 19 and 500. In that case you will consider only 300 and 200 sq ft offices can make 500sq ft and hence 19 office of each 200sq ft & 300sq ft. But there are many options which should be considered:

such as
a) 45 * 200 sq ft = 9000 and 200 sq ft + 300sq ft = 9500 Sq ft.
b) 30 * 300 sq ft = 9000 and 200 sq ft + 300 sq ft = 9500.
c) 26 * 350 sq ft = 9100 and 200 sq ft * 2 = 9500 sq ft.
and so on..

So A is insufficient.

B. Option B is insufficient as it mentions the number of 350 sq ft office and not other offices.

Combining both statements:
350sq ft *10 = 3500 , we are left with 6000 sq ft. We can get 6000 sq ft from either 200 sq ft * 30 or 300 sq ft * 20 and also some combination of 300 & 200. So both statements together are not sufficient.

Answer (E)