(b) 1440
There are 9 people, 6 gentlemen and 3 ladies. If each gentleman has to have a lady by his side, the seating arrangement can be done as shown below:
This can be done in 5! (Gentlemen) × 3! (Ladies) = 720 ways.
The arrangements can also be made in opposite direction. So,
Total number of required arrangements = 2 × 720 = 1440.