Explanation:
- The phrase “Of all the solutions proposed” indicates a comparison among many (superlative degree).
- For superlative degree, we use “most” before the adjective.
Sentence:
Of all the solutions proposed, this one is by far the most cost-effective.
Why other options are incorrect:
- more cost-effective → used for comparison between two things (comparative degree), not many
- cost-effectiver → incorrect spelling
- much cost-effective → grammatically incorrect
Extra facts:
- Positive degree: cost-effective
- Comparative degree: more cost-effective
- Superlative degree: most cost-effective
✔ Final Answer: most cost-effective