Final answer:
D. reduced. An organic molecule that gains hydrogen atoms is said to be reduced, following the principles of redox reactions where reduction is characterized by the gain of hydrogen or electrons.
Step-by-step explanation:
When an organic molecule gains hydrogen atoms, it is said to be reduced. This is understood in the context of oxidation-reduction (redox) reactions where reduction is characterized by the gain of hydrogen atoms or electrons, whereas oxidation involves the loss of hydrogen atoms or electrons. For example, when acetaldehyde (CH3CHO) gains hydrogen to form ethanol (CH3CH2OH), it is considered to have been reduced because hydrogen atoms have been added to it.
Oxidation and reduction can be defined in terms of the gain or loss of hydrogen atoms. If a molecule adds hydrogen atoms, it is being reduced. If a molecule loses hydrogen atoms, the molecule is being oxidized. For example, in the conversion of acetaldehyde into ethanol CH3CHO, hydrogen atoms are added to acetaldehyde, so the acetaldehyde is being reduced:
CH,CHO+H,→CH,CH,OH