Cagan's second discovery finding concerns the solution itself. Which three properties does he require it to have?

The principle — the second finding is about the solution, and it names three properties rather than one.

Why the key is correct — Cagan writes that Second, you need to discover a product solution to this problem that is usable, useful, and feasible. Three separate properties: people can work it, it does something worth doing, and the team can actually build it.

Why the others are wrong — desirable, profitable and defensible describe a business case rather than a discovered solution. Fast, reliable and available describe how a built product runs, which is a later question. And treating the three as one word loses exactly what the split is for, since a solution can be effortless and useless at the same time.

Remember this — usable, useful, feasible: three tests, not one.

Sources — Marty Cagan on product discovery, Silicon Valley Product Group.