Final answer:
The form not used to make a plural in French is -aux.
Step-by-step explanation:
French Plurals
In French, there are different forms used to make a plural depending on the word:
a) -s: This form is used for most words ending in a consonant, such as livres (books).
b) -x: This form is used for words ending in -au, -eau, -eu, or -ou, such as châteaux (castles).
c) -aux: This form is used for a small group of words like cheveux (hair).
d) -en: This form is used for a few words like yeux (eyes) and gants (gloves).
Therefore, the form that is NOT used to make a plural in French is c) -aux.