• Statement (A) is correct: Allosteric enzymes exhibit sigmoidal (S-shaped) saturation curves rather than the classical hyperbolic curves described by the Michaelis-Menten equation.
• Statement (B) is incorrect: Allosteric enzymes are virtually always multimeric / oligomeric (composed of multiple interacting subunits), not monomeric.
• Statement (C) is incorrect: Substrate binding across multiple active sites is characterized by cooperativity (conformational coupling where binding of one substrate molecule promotes or hinders subsequent substrate binding), rather than independent or non-cooperative binding.
• Statement (D) is correct: Allosteric modulators (effectors) bind reversibly to distinct allosteric sites, inducing conformational changes that either stimulate (positive allosteric effectors / activators) or diminish (negative allosteric effectors / inhibitors) catalytic activity.
• Thus, statements (A) and (D) only are correct (Option 4).

Option analysis:

• When $[S] \ll K_m$: The denominator approximates $K_m$, so $v \approx \frac{V_{\max}}{K_m}[S]$, obeying first-order kinetics -> (II).
• When $[S] \gg K_m$: The denominator approximates $[S]$, so $v \approx V_{\max}$, following zero-order kinetics independent of $[S]$ -> (I).
• When $[S] = K_m$: $v = \frac{V_{\max}[S]}{2[S]} = \frac{V_{\max}}{2}$, velocity equals one-half of maximum velocity -> (IV).
• When $[S] \approx [E_T]$: The fundamental Michaelis-Menten assumption ($[S] \gg [E_T]$) is violated because a significant fraction of substrate is enzyme-bound; thus standard $K_m$ and $V_{\max}$ do not describe the kinetics -> (III).