"Drop of nectar" is already in singular form.
The noun 'nectar' is an uncountable noun, as a word for a substance. For example, you could have "one bottle of nectar" or "a million bottles of nectar".The plural form of uncountable nouns for substances is reserved for 'types of' or 'kinds of'. For example, "it's made from the nectars of four different flowers."
The singular form of the demonstrative pronoun 'these' is this.
The singular form of "that" is "it."
it is already in its singular form.
The singular form is cuff.
"Stratum" is the singular form of stratum. The plural form is "strata".
The form hypothesis is the singular form. The plural form is hypotheses.
A singular form of dice is a die
The singular form of feet is foot.
The singular form of Oases is, Oasis.
The singular form of chromosomes is a chromatid.
"Wash" is the singular form of "wash."