The plural is heads of state
The plural form of state is states.
Head lice is actually already the plural form. The singular is head louse.
The plural form of city-state is city-states.
Secretaries of State
There is no plural form for the noun dismay. Dismay is a state of being, you are either in that state or you are not. When you need to say that a number of people are in the state of dismay, you use the adjective form, the dismayed people, or the verb form, they were dismayed.
state attornies general
Guilt is an uncountable noun for a state or feeling. It has no singular or plural.
Hairs. As in, the hairs on my head are aqua colored.
The plural form for the noun zoo is zoos. The plural possessive form is zoos'.Example: The zoos' inspections in our state are done on annual basis.
The plural form of the noun sheriff is sheriffs. The plural possessive form is sheriffs'.example: All sheriffs' training and qualification is done at the state level.
You should write "Their hats on their heads" because it matches the plural form of "hats" with the plural form of "heads".
The plural form of candidate is candidates.