Critical reasoning — identify implicit assumptions: Statement: “Whenever you have any doubt on this subject, you may refer to the book by Enn and Enn.” Assumptions: I. The book by Enn and Enn is available for consultation. II. There is no other book on this subject.

Introduction / Context:
Advice about reference material typically presupposes availability and usefulness. The speaker suggests consulting a specific book whenever doubt arises. We must extract the necessary assumptions for this advice to be meaningful.

Given Data / Assumptions:

• Directive: Refer to the book by Enn and Enn when in doubt.
• Assumption I: The book is accessible (in library, store, or personal copy).
• Assumption II: No other book exists on the subject.

Concept / Approach:
For the recommendation to work, the referenced book must be obtainable; otherwise the advice is futile. However, exclusivity is not necessary: the book can be recommended even if many other books exist.

Step-by-Step Solution:

Verification / Alternative check:
Consider that academic recommendations usually highlight a reliable source among many, not necessarily the only one. The instruction “you may refer to …” signals permissive guidance, not exclusivity.

Why Other Options Are Wrong:

• Only II / Either / Both: Incorrect because the existence of alternatives does not weaken the advice.
• Neither: Overlooks the necessity of availability.

Common Pitfalls:
Equating “refer to X” with “refer only to X,” and forgetting that availability is the critical practical assumption.

Final Answer:
Only assumption I is implicit