The singular form for the plural noun hobbies is hobby.
The noun hobby is singular; the plural form is hobbies.
The word hobby is a singular noun. The plural form is hobbies.
The phrase "hobbies of my friend" can make a sentence sound awkward and makes the sentence unduly longer. You can shorten it to "my friend's hobbies", with friend being a singular noun turned into a singular possessive noun.
The noun 'hobbies' is the plural form of the singular noun 'hobby', a common, abstract noun; a word for an activity done regularly in one's leisure time for enjoyment; a word for a concept; a word for a thing.
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.