The plural form for the noun auto is autos. The plural possessive form is autos'.
Example: Both of the autos' owners had let their insurance lapse.
The plural form is auto and the plural possessive is autos' (e.g. The autos' steering wheels were found to be defective.)
Singular possessive: secretary's Plural: secretaries Plural possessive: secretaries'
The plural is relatives. The plural possessive is relatives'.
The plural is lads. The plural possessive is lads'.
The plural form is echoes. The plural possessive is echoes'.
The plural form is suffixes. The plural possessive is suffixes'.
The plural form is Mexicans. The plural possessive is Mexicans'.
The plural form is stepsisters. The plural possessive is stepsisters'.
The plural form is inches. The plural possessive is inches'.
The plural possessive of "Father" is, "Fathers"
The plural possessive of bandit is bandits'.
Studies' is the plural possessive of study.