Fill in the blank choosing the correct option.
The other boys or Henry ______ to blame

In the given fill-in-the-blank, the 'is' is grammatically correct.
When two subjects in a sentence are connected with ''either...or, not...only, neither...nor, no...or, not... or, or, then verb should be used according to the nearest subject.
In the given sentence 'Henry' is the nearest subject. And it is singular.
Therefore, a singular helping verb should be used.