The singular form is parent-in-law.
A parent-in-law may be a father-in-law or a mother-in-law.
The singular form of the demonstrative pronoun 'these' is this.
The singular form of "cuffs" is cuff.
The singular form of "stratum" is "stratum." The word does not change in its singular form.
The singular form of feet is foot.
The singular form of "dice" is "die."
The singular form of "oases" is "oasis."
The singular form of "wash" is "washing."
The singular form of vixen is "vixen." Both the singular and plural form of this word are the same.
The singular form of "curricula" is "curriculum."
"Annex" is the singular form. The plural form would be "annexes".
The singular form of "hypothysis" is "hypothesis."
The singular form is ombudsman.