この薬を飲み___すれば、すぐ良くなりますよ。
Q. Which completes 'as long as you just take this medicine'?
Translation
As long as you take this medicine, you'll get better soon.
Explanation
Masu-stem + さえすれば means 'if you just do X (that alone suffices)': 飲みさえすれば. だけ doesn't form this conditional idiom with すれば.