Answer
lights
Background Information
What is a noun?
A noun is a person, place, thing, or idea
How do you know if a noun is plural?
Generally, if a noun is plural it will have an -s or -es at the end of the word. There are a few exceptions (ie octopi vs octopus; cacti vs cactus; die vs dice; fish vs fish; deer vs deer), although those are not relevant to this question
Answer Walkthrough
First, let us identify all the nouns in the given sentence by underlining them:
The city's lights shone brightly on the horizon last night.
Our nouns are:
- city's
- lights
- horizon
- night
Now let's describe what kind of noun each is (person, place, thing, or idea)
- city's = place
- lights = thing
- horizon = place
- night = thing
Remember, plural nouns will end in -s or -es. Which of the nouns above end in -s or -es?
Let's analyze each noun individually:
City's
This word ends in an -s however it follows an apostrophe ('). Unless the apostrophe (') follows the -s the word is not plural (moms', ducks', etc). Instead, an apostrophe (') before an -s can indicate one of two things. It could indicate a (1) contraction (ie it's = it is) or that the noun is (2) possessing something.
How do we know which is the case? Context is key, but if you are still unable to figure out, you can test it as such:
(1) Does 's indicate a contraction? To see this try removing the contraction and substituting it with the whole word being contracted (is). Does the sentence still make sense?
The city is lights shone brightly on the horizon last night.
This sentence does not make sense, thus we can conclude that it does not indicate a contraction
(2) Does 's indicate a possession? To see this try to identify what the noun is possessing
The city's lights shone brightly on the horizon last night.
The city owns the lights, thus the 's indicates a possession
City's is NOT a plural noun
Lights
Lights ends in an -s and do not contain an apostrophe ('). In the context of the sentence, it makes sense that there are multiple. Thus, lights is the plural noun of this sentence