If an enzyme has a low Km, it means:
The enzyme has high affinity for the substrate is the accurate response regarding enzymatic activity or regulation described in this question. Enzymes are biological catalysts that accelerate reactions by lowering activation energy through specific substrate binding and transition state stabilization. In the context of Enzyme Kinetics, The enzyme has high affinity for the substrate plays a specific catalytic or regulatory role determined by its active site configuration and mechanism of action. The other options (The enzyme has low affinity for the substrate, The enzyme cannot reach Vmax, and The reaction is independent of substrate concentration) are either different enzymes with distinct substrate specificities, act through different mechanisms, or are involved in separate metabolic pathways.
Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 6