Answer: C. Reacting one mole of the hydrocarbon with exactly one mole of hydrogen means the molecule has one carbon-carbon double bond (one degree of unsaturation). \(\text{C}_6\text{H}_{10}\) has two fewer hydrogens than the saturated alkane \(\text{C}_6\text{H}_{14}\), so it has one double bond and adds one mole of hydrogen.
The others (\(\text{C}_2\text{H}_6\), \(\text{C}_3\text{H}_8\), \(\text{C}_7\text{H}_{16}\)) are saturated alkanes and do not react.
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