Who decides a rules question in golf?

• The Committee is the person or group in charge of the competition or the course. It sets the boundaries, the terms of the competition and any Local Rules.
• A referee's ruling is final for the player. There is no appeal to the Committee, though the referee may choose to ask for a second opinion.
• The Committee's ruling is final. If it cannot decide, it may refer the question to the Rules of Golf Committee of the R&A.
• No referee near: the player must not delay play. In match play the player tells the opponent that a ruling will be asked for later, and must do so before either tees off on the next hole.
• Two balls, stroke play only: a player unsure of the right procedure may finish the hole with two balls, should say beforehand which is to count, and must report it to the Committee before returning the scorecard. Not reporting means disqualification.
• Video evidence is limited by the "naked eye" standard. Facts that could not reasonably have been seen with the naked eye are disregarded.
• A wrong ruling is corrected if it is still possible. If a referee wrongly authorises a breach, the player is not penalised.
• After a stroke-play competition has closed, penalties are not normally added. Disqualification is still possible for a knowingly wrong score or serious misconduct.
• A situation the Rules do not cover is decided by the Committee in a way that is "reasonable, fair and consistent" with similar cases.

Play has to go on, so somebody's decision must be final on the day. The two-ball procedure lets a stroke-play round continue without a guess becoming a penalty.