IMO E

explnantion;
4 tratctors plows in 12 hr
so in 1Hr four tractor can plow
1/12..field
so 6 tractor can plow
4 ~ 1/12
6 ~ X
so X = (6 * 1/12)/4
X = 6/48
X = 1/8
so in 1 hr 1/8 field to Plow complete field it will take 8 hrs