Fill the correct degree : Of all the solutions proposed, this one is by far the______.

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