Explanation:

part of field cultivated by A in 1 day = 2/(5×6)=1/15

part of field cultivated by B in 1 day = 1/(3×10)=1/30

part of field cultivated by A and B together = 1/15+1/30=3/30=1/10

4/5 part of field cultivated by A and B together in  = (4/5)/(1/10) days = (4×10)/5=8 days.

Hence, option 3 is the correct answer.