The function
Autonomy-driven behavior is a small human insisting they exist as a separate person. Kids are told what to do all day; the pushback is often the pressure valve.
What works
Offer two real choices you can live with ('red cup or blue?'). Hand back the HOW while you hold the WHAT. Collaborative Problem Solving (Ross Greene's model) is built for this — you solve the problem WITH them, not TO them. Save your no's for the ones that actually matter.
Parenting styles that help
Authoritative (firm + warm + flexible), Collaborative Problem Solving, and RIE-influenced approaches all lean into shared control. Democratic parenting tends to fit this kid best.
What backfires
'Because I said so,' ultimatums, sarcasm, public power struggles. Every corner you back them into forces them to choose defiance to keep their dignity.