Viscosity index (VI) reasoning: pick the single wrong statement from the following about multigrade oils and base stock types.

Introduction:
Viscosity index (VI) indicates how strongly a lubricant’s viscosity changes with temperature. High VI oils keep viscosity more stable across temperature swings, a desirable trait for automotive and industrial lubrication.

Given Data / Assumptions:

• Paraffinic vs naphthenic base oil behavior.
• Role of multigrade formulations and VI improvers.
• Definition of VI regarding temperature sensitivity of viscosity.

Concept / Approach:
By definition, a high VI corresponds to a relatively small change in viscosity with temperature. Multigrades attain higher VI using polymeric VI improvers. Paraffinic oils tend to have higher VI; naphthenic have lower VI. Therefore, the statement claiming “high VI means a large change” is wrong.

Step-by-Step Solution:
Confirm a: multigrades use VI improvers → high VI → correct.Confirm b: paraffinic base stocks → higher VI → correct.Confirm c: naphthenic → lower VI → correct.Assess d: contradicts VI definition → this is the wrong statement.Assess e: VI improvers elevate apparent VI → correct.

Verification / Alternative check:
Lubrication handbooks define VI such that higher VI equals better viscosity stability with temperature; OEM specs for multigrades target high VI using polymer modifiers.

Why Other Options Are Wrong (as picks):
a, b, c, e align with established lubricant science; only d misstates the concept.

Common Pitfalls:
Reversing the meaning of VI, or thinking multigrade just means a blend of two grades without polymeric VI improvement.

Final Answer:
High viscosity index means a large change in viscosity with change in temperature.