A t statistic is defined as some statistic minus a hypothesized value, for the numerator, which is then divided by estimated standard error of the statistic for the denominator. The correct option is: estimated standard error of the statistic
The t statistic is used to determine if the difference between the observed sample mean and the hypothesized population mean is statistically significant, taking into account the variability in the sample mean. If the absolute value of the t statistic is large, it suggests that the observed difference is unlikely to have occurred by chance alone.
So, to answer your initial question, the denominator of the t statistic is the "estimated standard error of the statistic." This accounts for the variability in the sample mean and allows you to standardize the difference between the observed sample mean and the hypothesized population mean.