Stifling is a verb. It is the -ing form of the verb stifle. Verbs don't really have a singular form like nouns but if you used stifle with a singular noun subject the verb form would be stifles.
She stifles her younger sister.
Stifle as a noun means the joint of a hind leg of certain four legged animals eg horse. But it is not used as a noun very often. If this is the meaning you are thinking of then the plural is stifles
The word 'duo' is the singular form; the plural form is duos.
Criterion is the singular form of criteria.
The word 'she' is in singular form. The plural for 'she' would be 'they'.
The singular possessive form is atlas's.
The word hypopharynx (one word) is the singular form; the plural forms are hypopharynges or hypopharynxes.
The singular form of the word flagella is flagellum.
The word 'duo' is the singular form; the plural form is duos.
That is the singular form; the plural form is those.
'Jackhammer' is the singular form.
The singular form is commando.
The word dog is the singular form.
"Biopsy" is the singular form, while "biopsies" is the plural form.
The singular form of the word "possum" is possum. The plural is possums.
Criterion is the singular form of criteria.
The singular form of "genera" is "genus".
The singular form for diagnoses is diagnosis.
The word 'she' is in singular form. The plural for 'she' would be 'they'.