Which of the above formulas belong to the alkanes С7Н12 С7Н14 С3Н6 С8Н14 С8Н16 С5Н8.

In chemistry, there is no such thing as alkalines. Most likely alkenes were meant here.
Alkenes (olefins, ethylene hydrocarbons) are acyclic unsaturated hydrocarbons containing one double bond between carbon atoms, forming a homologous series with the general formula CnH2n.
alkenes C5H8 C7H14 C7H12 C3H6
isomers of alkenes С8Н14 С8Н16



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