**Core Concept**
Isoenzymes (or isozymes) are variants of the same enzyme that catalyze the same reaction but differ in their amino acid sequence, structure, and expression in different tissues. They are often composed of multiple subunits and exhibit distinct kinetic and physical properties despite catalyzing identical biochemical reactions.

**Why the Correct Answer is Right**
Isoenzymes possess different **Km values** due to variations in substrate affinity, reflecting differences in enzyme-substrate interaction. They are typically **multimeric complexes**, composed of multiple polypeptide chains (e.g., lactate dehydrogenase with H, M, and B subunits). Additionally, they show **different physical properties** such as molecular weight, electrophoretic mobility, and stability due to structural differences. Thus, all listed features are accurate.

**Why Each Wrong Option is Incorrect**
Option A: While different Km values are possible, this is not always true for all isoenzymes—some may have similar Km values. However, the statement is still valid in many cases, so it's not a strong reason to reject it. But since the question says "true about isoenzymes," and Km variation is a recognized feature, this is acceptable.
Option B: This is correct—most isoenzymes are multimeric. The statement is valid and essential to understanding their structure.
Option C: Physical properties such as molecular weight and charge vary due to structural differences, making this true.

(Note: Since all statements are correct, "D. All of these" is the best choice.)

**Clinical Pearl / High-Yield Fact**
In clinical diagnostics, isoenzymes like LDH and CK-MB are used to identify tissue-specific damage (e.g., myocardial infarction), where different isoenzymes appear in specific organs due to differential expression.

✓ Correct Answer: D. All of these