Select the most appropriate option to fill in the blank.
Every year, some boys ________ in the middle of the course due to their family or financial problems.

Option A: drop off means to take someone or something to a particular place, usually by car, as you travel to a different place

Option B: drop by means to pay a brief, casual visit

Option C: drop out means to withdraw from participation or membership

Option D: Drop-in means to come for a visit, especially without having received an invitation for a specific time

Of all the given options, only option C fits in the context of the sentence, implying that every year, some boys, stop their participation or leave in the middle of the course. 

Therefore, Option C is the correct answer.