verb agreement - Use of the singular or plural "is" or "are" in ambiguous situations

In this sentence:

The only exception are questions that are narrow enough that they can
be reasonably answered definitively with one or two possible
solutions.

Should it be "The only exception is" because "exception" is singular, or "The only exception are" because "questions" is plural?

Answer

The default is that the verb agrees in number with its subject, so The only exception is . . .
If that sounds awkward, you can write The only exceptions are . . . , which is probably preferable anyway, given that questions is plural.