A noun that has an irregular plural a. adds the letter s to ...

[Jipu 123]

A noun that has an irregular plural
 
  a. adds the letter s to the singular form.
 b. adds the letters es to the singular form.
 c. changes the word itself.
 d. keeps the same form for both singular or plural.

Question 2

Singular or plural nouns
 
  a. refer to whether a noun is singular or plural in number.
 b. can usually be changed in number by adding or deleting -s or -es.
  c. sometimes use the same form for either singular or plural.
 d. all of the above

[Jbrasil]

Answer to Question 1

c

Answer to Question 2

d

[Jipu 123]

Thanks for the timely response, appreciate it

[softEldritch]

Excellent