I was thrown off too. The documentation is inconsistent.
At first I found the capacity limitations page that indicates 50 instances on a stand-alone server for all SQL Server editions except for Workgroup. Workgroup supports a maximum of 16 instances per computer.
Since this didn't match any of the answers I did a little more digging and found the following link that indicates 50 for Enterprise, and 16 for Standard and other editions.
I lucked out getting the right answer only because 50 and 50 wasn't one of the choices. Anyone even have this many instances installed to know the true answer?