In $C_4$ plants (Hatch-Slack pathway), the primary carboxylation reaction occurs in mesophyll cells where $\text{CO}_2$ (as $\text{HCO}_3^-$) is fixed by PEP carboxylase to form oxaloacetic acid (OAA, a 4-carbon compound). OAA is rapidly reduced to malic acid (in NADP-ME species like maize and sugarcane) or transaminated to aspartic acid (in NAD-ME and PEP-CK species). Tartaric acid is not a product of photosynthesis.

• Statement (A) is correct: More than 136 gibberellin forms ($GA_1$ to $GA_{136}$) have been identified from plants and fungi (well exceeding 50 forms).
• Statement (B) is correct: Gibberellins synthesize $\alpha$-amylase and hydrolytic enzymes to overcome seed and bud dormancy.
• Statement (C) is incorrect: Gibberellins delay senescence (especially in leaves and fruits, e.g., delaying rind senescence in citrus), they do not hasten it.
• Statement (D) is incorrect: Water uptake is primarily a physical-osmotic transpiration-driven process regulated by stomata (influenced by ABA), not directly hastened by GA.

In $C_4$ plants, atmospheric $\text{CO}_2$ is initially fixed in the mesophyll cytosol by Phosphoenolpyruvate (PEP) carboxylase using phosphoenolpyruvate ($PEP$) and bicarbonate ($\text{HCO}_3^-$) to form oxaloacetate ($OAA$). PEP carboxylase has a very high affinity for $\text{HCO}_3^-$ and lacks oxygenase activity, preventing photorespiration.