Which of the following is the most abundant protein in the biosphere?

Correct option is C

The correct answer is (c) RuBisCO
Explanation:
• RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyses CO₂ fixation in the Calvin cycle.
• It is present abundantly in chloroplasts of plants, algae and cyanobacteria.
• Because of its ubiquity in photosynthetic organisms, it is the most abundant protein on Earth.
• RuBisCO can constitute a large fraction (up to ~50%) of leaf soluble protein in many plants.
• NCERT Class XI Biology and standard biochemistry texts confirm this.
Information Booster:
• Collagen is the most abundant protein in animals, but not in the biosphere overall.
• RuBisCO’s abundance is linked to the global scale of photosynthetic biomass.
Additional Knowledge:
GLUT-4 (a) — An insulin-regulated glucose transporter in muscle/adipose.
Collagen (b) — Abundant in animals (connective tissue), but not global-most.
Myosin (d) — Major muscle protein, not globally most abundant.