Majors'.
The singular possessive is cost's (e.g. The cost's increase was staggering.)The plural is costs. The plural possessive is costs' (apostrophe only).(e.g. As the prices of raw materials and energy rose, the costs' major impact was on small businesses.)
Major General's would be singular possessive. Such as: The Major General's decision could change the lives of his men. Plural possessive would be Major Generals', with the apostrophe after the s.
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 Mexicans. The plural possessive is Mexicans'.
The plural form is echoes. The plural possessive is echoes'.
The plural form is suffixes. The plural possessive is suffixes'.
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"
Studies' is the plural possessive of study.