It is the best question a client can ask and most advisers answer it badly, by arguing with the premise instead of conceding it.
Low-cost index exposure is a genuinely excellent default. It is cheap, transparent, and does not depend on anyone being right about anything.
An adviser who cannot say that plainly has already lost the conversation, because the client knows it is true.
If the answer is security selection, the index question is a serious challenge and it deserves a serious answer.
If the answer is everything else — the allocation, the tax management, the plan, the behaviour — then the index question is about one component of a larger service.
Which index. How much of it. What else is held alongside. When money goes in and comes out. What happens in the account that is taxable versus the one that is not. Whether the client stays invested.
Those are all decisions, and none of them are made by choosing a low-cost fund.
Buying the index answers what to hold. It does not answer how much, alongside what, in which account, or whether you keep holding it.
The most common claim for advice is that advisers prevent clients from abandoning strategies at bad moments.
There is a real basis for that. It is also frequently quantified with more precision than the evidence supports, and inflated numbers get repeated because they are useful. Make the argument qualitatively and you can defend it.
For a client with a simple situation, a long horizon, a single account type, and the temperament to leave it alone, a low-cost index portfolio is a very good answer.
Saying so is not a failure of nerve. It is the fastest way to be trusted by the clients for whom the answer is different.
If a client is paying for advice, the advice has to be visible. Where it is not, the index question is not a challenge to be handled. It is correct.