Positive phototropism: shoot growth bends toward unilateral light source

Steps:

• Recall positive phototropism as auxin-driven bending of shoots toward light.
• Examine setups: unilateral light causes bending in untreated shoots (2, 3, 4); controls or blocks prevent it (1, 5).
• Identify response: shoots 2 (exposed tip), 3 (partial cover allowing light detection), and 4 (intact with light) bend toward light.
• Confirm non-response: 1 (diffuse light, no direction) and 5 (tip covered, auxin redistribution blocked) show no bending.

Why B is correct:

• Shoots 2, 3, and 4 detect unilateral light via auxin accumulation on shaded side, causing positive bending per Darwin's phototropism experiments.

Why the others are wrong:

• A omits 3 and 4, which also bend toward light.
• C excludes 3 and 4, ignoring their valid responses.
• D misses 3, where partial exposure still enables phototropism.

Final answer: B