Discontinuous variation features distinct categories without intermediates

Steps:

• Recall discontinuous variation involves traits in clear, separate groups, often due to single genes.
• Examine each option for distinct vs. gradual changes.
• Identify blood group as having fixed types (A, B, AB, O) with no blends.
• Confirm others show continuous ranges influenced by multiple genes and environment.

Why B is correct:

• Blood groups follow Mendelian inheritance with discrete alleles, producing categorical phenotypes without intermediates, per genetic laws.

Why the others are wrong:

• A: Height varies continuously along a spectrum due to polygenic and environmental factors.
• C: Hair colour shows gradations (e.g., shades of brown) and is polygenic, not strictly categorical.
• D: Mass (weight) changes gradually and continuously with diet and growth.

Final answer: B