Simplify: 23 - [3 - {9 - (4 + 5)}]

The correct option is 20.

To simplify the given numerical expression, we follow the standard order of operations (often remembered by acronyms like PEMDAS or BODMAS). This rule states that operations inside brackets or parentheses must be performed first, starting from the innermost groupings and working outward.

The given expression is:

$23-[3-\{9-(4+5)\left\}\right]$

Step 1: Simplify the innermost parentheses $(4+5)$.

Calculate the sum inside the round brackets:

$4+5=9$

Substitute this value back into the expression:

$23-[3-\{9-9\left\}\right]$

Step 2: Simplify the curly braces $\{9-9\}$.

Calculate the difference inside the curly brackets:

$9-9=0$

Substitute this value back into the expression:

$23-[3-0]$

Step 3: Simplify the square brackets $[3-0]$.

Calculate the difference inside the square brackets:

$3-0=3$

Substitute this value back into the expression:

$23-3$

Step 4: Perform the final subtraction.

$23-3=20$

Thus, the simplified result is 20.