Confidence intervals; standard error vs standard deviation

Hello, please excuse my English

Could someone tell me why we don’t use the confidence interval formula (point estimate ± (reliability factor × standard error)) here and instead we use the standard deviation directly?

The question is from schweser l1 practice exam4, QM, Q30.

I assume we should use standard error in this situation because we want to estimate the population mean.

Best regards,
Mayuko

They have a sample of means, not a sample of individual returns.

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where did the square root of n go?