To isolate something or someone is to

• A. to seclude someone or something
• B. to place something in quarantine
• C. to separate something or someone
• D. to join together

Answer: (A)

Isolate means to set apart from others, keeping something or someone away from the rest. Seclude fits best because it specifically conveys withdrawing from general contact, often with an element of privacy or confinement. That sense of being kept apart captures the core idea of isolation in many contexts. Placing something in quarantine is a specific, health-related way of isolating, so it’s too narrow. To separate is close but doesn’t stress the privacy or confinement aspect as strongly, and to join together is the opposite of isolating.