The correct answer is D. no article.
The word "idea" is a non-count noun, which means it does not have a plural form and does not take a determiner (such as an article) unless it is modified by a word that specifies the kind of idea. In the sentence "That's a good idea," the word "idea" is modified by the adjective "good," so it takes the article "a." However, in the sentence "That's a good idea," the word "idea" is not modified by any adjective, so it does not take an article.
Here are some other examples of non-count nouns:
- water
- sand
- air
- love
- happiness
In each of these examples, the noun does not have a plural form and does not take a determiner unless it is modified by a word that specifies the kind of noun. For example, we can say "I like water" or "I like to swim in the water." However, we cannot say "I like a water" or "I like the water."
I hope this explanation is helpful! Let me know if you have any other questions.
