Variations in the reactive properties of different organic molecules are most closely associated with
a. the number of carbon atoms composing the molecule's skeleton
b. the presence or absence of double bonds
c. the presence or absence of functional groups
d. the orientation of the carbon skeleton, as either ringed or linea

c. the presence or absence of functional groups

Explanation:

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In organic chemistry the varied types of compounds and their chemical reactions depend on the present functionalities, for instance, alcohols (R-OH) react with hydracids to yield alkyl halides (R-X) but if an anhydride (R-COOC-R') reacts with a hydracid it yields an alcohol and an acyl halide (R-COX), thus, we could find several different chemical reactions that are mainly led by the reacting functional group rather than by the amount of carbons, double bonds or the orientation of the carbon skeleton, which are factors affecting the reactivity but not as far as how the presence of functional groups does it.

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