Explanation:

Correct Option 1: 2400

We have 12 km i.e. 12000 m race in a circular track of the length of 1200 m.
Number of round to complete the race = 12000/1200 = 10 rounds
Now,
Time taken by A to complete the first round = 300 seconds
For 10 rounds it will take 300 x 10 = 3000 seconds
Now,
When A and B both start running,
First meet will take place in L.C.M (300,400) seconds = 1200 sec
Next meet will take place = 1200 x 2 = 2400 seconds
For next meet,
Time would be 3600 seconds, which is Not possible as A completes the race in 3000 seconds
But, to complete 10 rounds A takes 3000 seconds. So, time cannot be more than 3000 seconds
So, faster person will meet the slower person last time at 2400 seconds
Hence, A will meet B last time at 2400 seconds.