Statement (I) is correct: Tillage operations produce ideal soil tilth when soil moisture is at optimum friability, which generally occurs at 25% to 50% depletion of available soil moisture (or 50% to 75% available moisture content).

Statement (II) is incorrect: Light (sandy) soils have a wider workable soil moisture range because they contain less clay and do not easily puddle or form clods. In contrast, heavy (clay) soils have a very narrow workable moisture range; if too wet they puddle and stick to the implement, and if too dry they become extremely hard and produce unmanageable massive clods.

Precision Agriculture follows a logical cyclical workflow:

• Assessing variability (D): Measuring spatial and temporal soil and crop variability using GPS, grid soil sampling, sensors, and yield monitors.
• Preparation of variability maps (B): Visualizing data and creating spatial prescription maps in GIS.
• Managing variability (C): Applying site-specific inputs (variable rate application - VRA of fertilizers, seed, water, and chemicals).
• Evaluation of precision agriculture (A): Assessing yield response, economic profitability, and environmental outcomes.

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.