Step 1 :The question is asking for the definition of the term 'standard error of the sample mean'. This term is a statistical concept that refers to the standard deviation of the distribution of sample means. It is a measure of how much the sample mean is expected to vary from the true population mean.
Step 2 :The options given are: a. The population standard deviation, b. The sample standard deviation, c. The standard deviation of the $x$-bar curve, d. Human error in the sampling process.
Step 3 :The standard error of the sample mean is not the same as the population standard deviation or the sample standard deviation, so options a and b are incorrect. It also does not refer to human error in the sampling process, so option d is incorrect.
Step 4 :The standard deviation of the $x$-bar curve, however, is a correct definition of the standard error of the sample mean. The $x$-bar curve is a distribution of sample means, and its standard deviation is the standard error of the sample mean.
Step 5 :Final Answer: \(\boxed{\text{c. The standard deviation of the } x\text{-bar curve}}\)