DNA Components and Hierarchy

Steps:

• Define allele: a variant form of a gene, which is a segment of DNA, not a carrier of DNA molecules.
• Evaluate statement 1: Incorrect, as alleles are sequences on DNA, not entities that carry DNA molecules.
• Evaluate statement 2: Correct, chromosomes are structures containing one long DNA molecule wrapped with proteins.
• Evaluate statement 3: Correct, a single DNA molecule in a chromosome is organized into multiple genes along its length.

Why D is correct:

• Statements 2 and 3 align with the definition of chromosomes as DNA-containing structures and genes as functional units dividing the DNA molecule.

Why the others are wrong:

• A includes incorrect statement 1.
• B includes incorrect statement 1.
• C includes incorrect statement 1.

Final answer: D