We have bought extra food _________________ our friends stay over.

The correct answer is 'in case'. 'In case' is used to express doing something as a precaution against something happening. 'We have bought extra food in case our friends stay over' means the extra food is a precaution in the event that friends visit. 'In case' introduces a conditional possibility. Option A 'if' is possible but less precise for showing precaution. Option B 'so' would need different structure. Option D 'as long as' introduces a condition rather than a precaution. 'In case' best captures the idea of preparation for a possible future event.