に
Particle
1. at (place, time), in, on, during
2. to (direction, state), toward, into
3. for (purpose)
4. because of (reason), with
5. by, from
限る, かぎる
Godan verb, Transitive
1. to restrict, to limit, to confine
Intransitive
2. to be restricted to, to be limited to, to be confined to
as ...は...に限る
3. to be best (for), to be the best plan, to be the only way (to)
暑い日は冷たいビールに限る。