Question E6C11:
In Figure E6-3, which is the schematic symbol for the NOT operation (inversion)?  

Choices:  
A: 2  
B: 4  
C: 5  
D: 6    

Correct Answer is  C: 5  

Correct choice: C (5)

Why 5 is correct:
- The NOT (inverter) symbol is a triangle with a small circle (bubble) on its output. The bubble indicates inversion.
- Functionally, it performs $Y = \overline{A}$ (output is the logical complement of the input).

Why the others are incorrect:
- Choice 2: This is a multi-input gate without an output bubble (e.g., AND or OR). No bubble means no inversion, so it is not NOT.
- Choice 4: Triangle without a bubble is a buffer. It performs $Y = A$, not inversion.
- Choice 6: Multi-input gate with an output bubble (e.g., NAND or NOR). While it includes inversion, it is not a single-input inverter.

Useful rules of thumb:
- A small circle (bubble) on a pin denotes inversion at that pin.
- Triangle symbol:
  - No bubble: buffer ($Y = A$)
  - Bubble on output: inverter/NOT ($Y = \overline{A}$)
- AND/OR families:
  - AND: flat left, curved right; NAND adds an output bubble: $Y = \overline{A \cdot B}$
  - OR: curved inputs; NOR adds an output bubble: $Y = \overline{A + B}$
- XOR has the OR shape with an extra curved line on the inputs (no bubble unless XNOR).