How do we calculate the standard error of the mean?

Sample standard deviation divided by square root of sample size

Square root of sample variance

Square root of population variance

Sample standard deviation divided by sample size

Which of the following description of the standard error of the mean is correct?

The standard error of the mean is the standard deviation of the sampling distribution of the mean

The standard error of the mean is the squared variance of the sampling distribution of the mean

Standard error is the sample standard deviation

Standard error is the same with population standard deviation

The sample standard deviation is 50, and the sample size is 400.