Frame the trade-off: "Multisite simplifies management when sites are similar — shared updates, shared users, one dashboard. But it creates a blast radius: a bad plugin update takes down every site. I recommend it for tightly related sites and separate installs for independent projects."
Multisite is appropriate when sites share users, themes, and plugins under one admin — for example, a university with departmental sites or a franchise with location pages. Separate installations suit sites with different hosting requirements, independent update cycles, or distinct plugin sets. Multisite implications: plugins can be network-activated or per-site, themes are network-enabled then activated per-site, and the database uses separate tables per site (with a shared users table). Strong candidates discuss: switch_to_blog()/restore_current_blog() for cross-site queries, the wp_is_large_network() performance consideration, domain mapping, and the risk that one plugin breaking affects all sites.
Tests architectural judgement. Candidates who recommend Multisite for unrelated sites or who have never used it both lack experience. Those who understand the shared-risk model and can articulate when centralised management outweighs the coupling are strong.