The average profit of B in the year 1998 = $4000$ The average profit of A in the year 1998 = $3000+X$, where $X$ is positive and not more than $500$ (by observation)

Difference between the average profit of B and A in the year 1998 = $4000 - (3000+X) = 1000 - X$ = positive.

Therefore, answer is C. $+Rs. 600$