grammar - "Responsible" as predicate adjective

"They are the only responsible for the situation"

"They are the only ones responsible for the situation"

"They are the only one responsible for the situation"

Which of these options is/are grammatically correct? There are many hits for all of them in Google.

I would've thought the third option is incorrect.

Answer

"They are the only responsible for the situation"

This sentence is ungrammatical because the is a definite article which can only modify nouns, and there is only an adjective present. You could, however, restructure it to be Only they are responsible for the situation.

"They are the only ones responsible for the situation"

Here they refers to a group of people, and ones agrees with this plurality.

"They are the only one responsible for the situation"

Though there has been some debate about it in the past, many people accept they as a gender-neutral third person singular pronoun, otherwise referred to as the singular they. Accordingly, one agrees with this.