My take

Conclusion- individuals cannot easily adopt those strategies unless a doctor or some other third party provides the warning by deception.

Assumption is that people cannot deceive themselves... only D matches

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Conclusion - Individuals need some third party or doctor to convince them, Only D seems relevant here

Conclusion: Individuals can't adapt unless doctor or a 3rd party gives warning

Let's say individuals can do it without doctor/ 3rd party. This means there are other tools to achieve deception.

therefore, My answer would be something which says that there is no other way apart from the doctor or 3rd party to achieve deception

a) contender
b) irrelevant
c) irrelevant
d) contender
e) irrelevant

Let's apply negation test to A and D. Clearly A can't break the argument as in absence of doctors, 3rd party can deceive
But incase of D, it completely shatters the argument.

Hence D

A smoker trying to quit is more likely to succeed if his or her doctor greatly exaggerates the dangers of smoking. Similar strategies can be used to break other habits. But since such strategies involve deception, individuals cannot easily adopt them unless a doctor or some other third party provides the warning.

Which one of the following is an assumption on which the argument depends?

My Take :-

Conclusion:- individuals cannot easily adopt strategies,unless a doctor or some other third party provides the warning
Prethinking - Individuals trust more on doctors or other third party than themselves , if any adopted strategy involved deception.

A. People tend to believe whatever doctors tell them.
Wrong - This is more than what we want here . people dont tend to believe whatever doctor tell them
B. Most of the techniques that help people quit smoking can also help people break other habits.
Wrong - This assumption doesnt impact the conclusion
C. The more the relevant danger is exaggerated, the more likely one is to break one's habit.
Wrong - Now where this co-relation is mentioned
D. People generally do not find it easy to deceive themselves.
Correct - more inline with prethinking
E. A doctor is justified in deceiving a patient whenever doing so is likely to make the patient healthier.
Wrong - No impact on conclusion
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assumption. It seems like author of question is looking for an assumption that is contrast to the question.