Identify an example of plasma as a state of matter.

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1. Definition: Plasma is an ionized gas containing free electrons and positive ions; a useful parameter is the degree of ionization $\alpha=\dfrac{n_{	ext{ion}}}{n_{	ext{total}}}$, which is significant (nonzero) in a plasma.

2. Evaluate options:
- (1) Blood: "plasma" here refers to the liquid component of blood (water + proteins), which is not an ionized gas, so it is not the plasma state of matter.
- (2) Dry ice: solid carbon dioxide, so it is a solid, not plasma.
- (3) Freon: a refrigerant that is a gas or liquid under typical conditions, not an ionized gas.
- (4) Neon sign bulbs: contain neon gas that becomes ionized by an electric discharge, producing free electrons and ions (a glowing plasma). This matches the definition.

3. Conclusion: The correct example of plasma as a state of matter is neon sign bulbs.

FINAL_ANSWER: d

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