Historical Rules · 8 March 2024
Rule 509.1a
Official source text from the 8 March 2024 Rules release
The defending player chooses which creatures they control, if any, will block. The chosen creatures must be untapped and they can’t also be battles. For each of the chosen creatures, the defending player chooses one creature for it to block that’s attacking that player, a planeswalker they control, or a battle they protect.