Final answer:
The modern motherboards generally utilize a 24-pin P1 connector to connect to the power supply.
Step-by-step explanation:
The answer to the question 'Modern motherboards use a 20- or 24-pin _______________.' is A. P1. This refers to the main power connector used to supply power to the motherboard from the power supply unit (PSU). Modern motherboards commonly use a 24-pin P1 connector, although some may still use a 20-pin version or include a 4-pin additional connector that can modify a 20-pin to function as a 24-pin connector.