Correct option (b) 6.30 p.m.
According to question,
Ganga begins at 9 am and she does 3 units/hours Saraswati begins at 10 am and she does 2 units/hours So by 11 am they complete 5 units
And now Ganga will complete 3 unit out of 4 units in 1 hr Now,
Rest 1 unit work done by Saraswati in = 1/2 hr