Answer (4) 300

solution depends on maximum load on the horse’s back when he starts

We will find the point where horse can transport 900 carrots. Let’s say this point is P. Horse will pick carrots twice from point A. First time horse will go to point P and come back. Hence horse will consume carrots in both direction. Next time horse will only go (will not come back). Remaining 900 carrot means he has consumed 900 carrots to reach point P. Effectively horse has traveled thrice the distance between A and P, and consumed 900 carrots. it means to travel from A to P horse consumed 300 carrots. Hence point P is 300 kms away. 

from P now B is 600 kms away. Horse will further eat 600 carrots. Hence remaining transported carrots are 300.