Final answer:
Monoamine oxidases are enzymes that target amines, specifically oxidizing monoamines. These include neurotransmitters with modified amino acid structures, making 'd) Amines' the correct substrate for these enzymes.
Step-by-step explanation:
Judging from the name, monoamine oxidases are enzymes that would likely have amines as a suitable substrate. The name indicates their function is related to the oxidation of monoamines, which are biogenic amines enzymatically made from amino acids. They possess amino groups but lack carboxyl groups. Examples of these are neurotransmitters like norepinephrine, which has a modified amino acid structure with a benzene ring replacing the original carboxyl group. Therefore, the correct answer to which substrate would be suitable for this class of enzymes is d) Amines.