This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
Standard error of mean is called as- 
Select and explain. 
A. Standard deviation, B. Mode, C. Median, D. Variable
A: Standard deviation
Answer: A: Standard deviation 
Explanation: Standard error is nothing else but the standard deviation of the whole population:
For a single sample measure of dispersion → SD
For whole population measure of dispersion → SE

"Standard error of a statistic is the standard deviation of the sampling distribution of the statistics"  — Handbook