From the Standard Deviation Formula $\frac { \sum ( x - \bar { x } ) ^ { 2 } } { n - 1 }$

From the standard deviation formula, $\frac { \sum ( x - \bar { x } ) ^ { 2 } } { n - 1 }$ identify the estimator.

A) s²
B) ?²
C) $\bar { x }$
D) s