Teresa Torres uses the term product trio. Who is typically in one?

The principle — the trio is named by the three perspectives it puts in the same room.

Why the key is correct — Torres writes that A product trio is typically comprised of a product manager, a designer, and a software engineer. One person who decides what gets built, one who shapes how it works, and one who knows what it would take to build it.

Why the others are wrong — swapping the designer for a second engineer drops the perspective the arrangement is built around. Three managers of the work are not the people doing it, so they cannot hold a shared view of the customer between them. And an organisation without designers still has the definition; what it lacks is the third member.

Remember this — product manager, designer, engineer: one of each, not three of any.

Sources — Teresa Torres on the product trio, Product Talk.