Which sentence has an error in a plural noun?
the smithsonian exhibit displays the inauguration gowns of many first ladies.
many coastal cities were affected by the hurricane.
good chefs bring their own knifes.

Third Option: Good chefs bring their own knifes.

It should be "Good chefs bring their own knives.

Explanation:

Knife is a noun which is made plural by replacing "fe" with "-ves" i.e "knives" from "knife"

Many nouns end with "f" sound are made plural by adding "-ves" at their end. But there are many other nouns ending with "f" sound but made plural by simply adding "-s" at the end.

There is no hard and fast rule for identifying whether to add "-ves" or simple "-s" and even if some rule is made, there appear many exceptions to such rules. So it is better to learn them by practice.

1: Words ending with -ff are made plural by simply adding -s, e.g;

cliff - cliffs

toff - toffs

scuff - scuffs

sniff - sniffs

2: Some words ending in f are made plural by adding -s in the end, e.g

roof - roofs

chef - chefs

chief - chiefs

oaf -oafs

3: Words which end in f or fe and are made plural by -ves

calf - calves

half - halves

knife - knives

leaf - leaves

loaf - loaves

life - lives

wife - wives

shelf - shelves

thief - thieves

yourself - yourselves

themselves

4: Some words can have both endings -ves or -s, e.g

scarf - scarfs/scarves

dwarf - dwarfs / dwarves

wharf - wharfs / wharves

handkerchief - handkerchiefs / handkerchieves

First and second options correctly use plural nouns.

b is the correct answer