時間___あれば、もっと旅行したいです。
Q. Which particle completes 'if only I had time'?
Translation
If only I had the time, I'd travel more.
Explanation
さえ〜ば is the fixed 'if only / as long as' pattern: 時間さえあれば. こそ emphasizes but doesn't form this conditional idiom, and しか needs a negative.