The electricity is . . . . . . than coal.

Answer: Option C

The sentence is comparing the cost of electricity with coal. In English grammar, when making a comparison between two things, we use the comparative form of an adjective.

Rule for Comparative Adjectives:
For short adjectives (one syllable or two syllables ending in -y), we add **"-er"** to form the comparative.

Base Form: cheap
Comparative Form: cheaper
Superlative Form: cheapest

Since the sentence is comparing **electricity** and **coal**, we need the **comparative form**, which is **"cheaper."**

Why Not the Other Options?
Option A: cheap – This is the base form and does not show comparison. The sentence requires a comparative adjective, so this is incorrect.
Option B: to cheap – This is grammatically incorrect. "To" should not be used before an adjective in this context.
Option D: cheapest – "Cheapest" is the **superlative form**, used when comparing more than two things. Since we are only comparing electricity and coal, "cheapest" is incorrect.

Thus, the correct answer is **Option C: cheaper**.