Monomers are single units that form polymers

Steps:

• Identify monomers as small molecules that link to form larger polymers like proteins, carbohydrates, or nucleic acids.
• Examine molecules 1–4 based on their structure: monomers have functional groups for bonding without repeating chains.
• Classify each: 1 is an amino acid (monomer), 2 is a monosaccharide (monomer), 3 is a nucleotide (monomer), 4 is a disaccharide (polymer).
• Select option listing all monomers: 1, 2, and 3.

Why A is correct:

• A identifies 1, 2, and 3 as monomers per the definition of simple building blocks for biopolymers.

Why the others are wrong:

• B includes 4, a disaccharide polymer, not a monomer.
• C omits 2, a monosaccharide monomer.
• D excludes 1 and 3, both monomers, and includes 4, a polymer.

Final answer: A