Explanation

The sentence "I was disappointed that I did so bad in the examination" contains a grammatical error specifically in part D, the word "bad." In standard English grammar, "bad" is an adjective, which should modify nouns. However, in this context, the word "did" requires an adverb to properly modify the verb. The correct adverb form here is "badly," which describes the manner in which the action (the examination performance) was carried out.

Thus, the correct phrasing should be "I was disappointed that I did so badly in the examination." This grammatical structure clarifies that the speaker is expressing dissatisfaction with the quality of their performance, which is essential to the meaning of the sentence.

In conclusion, the error in part D (using "bad" instead of "badly") is what makes this part of the sentence incorrect, thus making "D" the right answer to the given question. This highlights the importance of choosing the correct part of speech to convey the intended meaning accurately.