Which of the following formula is used in calculating standard deviation?

where $\bar{x}$ is the mean $\bar{x} = \dfrac{\sum x_i}{N}$.

$\sqrt{\dfrac{\bar{x} - x}{\sqrt{N}}}$.
This lacks the sum $\sum$, squared differences $(x_i - \bar{x})^2$, and has incorrect denominator $\sqrt{N}$. Incorrect.

$\sqrt{\dfrac{\sum (x_i - \bar{x})}{N}}$.
This averages the differences $(x_i - \bar{x})$ without squaring them. Incorrect (this resembles mean absolute deviation).

$\sqrt{\dfrac{\sum (x_i - \bar{x})^2}{N}}$.
This matches the standard deviation formula exactly. Correct.

$\sqrt{\dfrac{\sum (x_i - \bar{x})^2}{N}}$ (OCR artifacts like "10 szu" likely irrelevant; appears identical to C, but context suggests possible sample variant—however, standard population uses $N$).
Option C is the clear match.

$\sqrt{\dfrac{(x - \bar{x})^2}{N}}$.
Lacks the sum $\sum$ over all data points. Incorrect.

C