Answer: Sample A
Reason:
The standard error of the mean has the formula
where
(greek letter sigma) is the standard deviation and n is the sample size.
As n gets bigger, the standard error
gets smaller, and vice versa. This assumes sigma is fixed. A larger sample indicates the error goes down to narrow in better on the population mean.
Based on what I mentioned, this tells us that sample A has the larger standard error since n = 350 is smaller than n = 500 for sample B.