Intelligibility is a non-count noun, so there is no plural form.
Another example of a non-count noun is understandable. Because you can not count understandable it will never appear in a plural form (understandables).
The plural form of intelligibility is intelligibilities.
The plural form of "do" is "do" and the plural form of "don't" is "don't." These words do not change in the plural form when used in a sentence.
The plural form of him, her, or it is them. (objective pronouns)
The plural form of "was" is "were."
The plural form of "I" is "we."
The plural form of "is" is "are."
Jerry V. Tobias has written: 'Massed versus distributed practice in learned improvement of speech intelligibility' -- subject(s): Intelligibility of Speech, Research, Speech, Intelligibility of
The plural form of him, her, or it is them. (objective pronouns)
Bridges is the plural form of bridge.
"Beliefs" does not have a plural form, as it is already plural. Beliefs is the plural form of belief.
There is no plural form for the word, countries. This word itself is a plural.
"Groceries" is the plural form of "grocery."
The plural form of "meter" is "meters."
The plural form of the demonstrative pronoun 'that' is those.
The plural form for the noun sergeant is sergeants; the plural possessive form is sergeants'.
It has no plural form.
The plural form of rung is rungs.
The plural form of parenthesis is parentheses.