United

You can buy it a seat on this airline.

United rules

A violin, guitar or other small instrument may be carried on in a hard case. In the bin it counts as your carry-on bag; under the seat, as your personal item.
US federal rule: the airline must accept it if it can be stowed and there is space when you board. There is no priority over other bags.
A seat can be bought for an instrument under 165 lb (75 kg). Call 1-800-864-8331; an agent must approve the item.
The instrument seat must be next to yours and the item stays belted in for the whole flight.
United Express flights use smaller aircraft with smaller bins.

United instrument policy

14 CFR 251.3: a small instrument must be accepted as carry-on if it can be stowed and space is available at boarding.
United policy: small instruments in a hard case are accepted on board.

https://united.com/en/ec/fly/baggage/cabin-seat.html