The opportunity cost refers to the loss of the next best alternative when a decision is made. In this case, the laborer had two choices: continue working as an agricultural laborer for ?400 per day or become a babysitter for ?700 per day.

Option 1: ?1,100 - This is not correct because there`s no reference to this amount in the question.

Option 2: ?700 - This is the income she would earn if she decided to be a babysitter, but it is not the opportunity cost.

Option 3: ?400 - This is her current income and not the cost of the lost opportunity.

Option 4: ?300 - This is the correct answer. The opportunity cost of continuing her job as an agricultural laborer is the difference between what she could earn as a babysitter (?700) and what she currently earns as an agricultural laborer (?400), which is ?300. This is the income she loses out on by not taking the babysitting job.