Which of the following is an environmentally friendly treatment that supports the preservation of flowers by countering ethylene gas?

MCP, or 1-methylcyclopropene, is an environmentally friendly treatment that effectively inhibits the effects of ethylene gas, which is a natural plant hormone responsible for the aging and ripening of flowers. Ethylene exposure can lead to faster senescence in flowers, ultimately reducing their shelf life and visual appeal. MCP works by blocking the receptors that ethylene binds to, thereby delaying the wilting process and extending the longevity of floral arrangements. Its targeted action helps maintain flower quality and supports sustainable practices in the floral industry.

The other options do not provide the same benefits. Citric acid is often used as a preservative or acidifier but does not specifically counter ethylene's effects. Chlorine bleach is used for sanitizing but can be harmful to flowers and does not address ethylene exposure. Vinegar solution may alter the acidity of water and can help to reduce bacteria but also does not specifically inhibit ethylene's effects on flowers. Thus, MCP stands out as the correct choice for supporting the preservation of flowers against ethylene gas.