ill in the blanks with a suitable option. They were very tired in the evening because they _______ on the farm all day.

The suitable option to fill in the blank is "worked."

The complete sentence reads: "They were very tired in the evening because they worked on the farm all day."

This sentence indicates that the subjects exerted physical effort throughout the day, which led to their tiredness in the evening. The use of "worked" effectively conveys the action they were engaged in, reinforcing the notion that a long day of laborious activity is the reason for their fatigue. The structure also follows the cause-and-effect format, where their prior actions (working on the farm) logically lead to their consequent state (being tired).