The correct answer is D. alkane, alkyne, alkyne.
An alkane is a hydrocarbon that contains only carbon and hydrogen atoms and has the general formula CnH2n+2. An alkyne is a hydrocarbon that contains only carbon and hydrogen atoms and has the general formula CnH2n-2. An alkyl group is a hydrocarbon group that contains one or more carbon atoms and is derived from an alkane by removal of one hydrogen atom.
The number of hydrogen atoms in an alkane, alkyne, and alkyl group is given by the following equations:
Number of hydrogen atoms in an alkane = n + 2
Number of hydrogen atoms in an alkyne = n – 2
Number of hydrogen atoms in an alkyl group = n
Therefore, the correct descending order of hydrogen atoms is alkane, alkyne, alkyne.