The following sentence has been divided into four segments. Identify the segment that has a grammatical error.

His energetic attitude / showed that he had / soundly slept / the previous night.

The correct answer is soundly slept.

Detailed Explanation:
Let us analyze the sentence structure segment by segment:

1. "His energetic attitude": This is the subject of the sentence and is grammatically correct.
2. "showed that he had": This introduces a subordinate clause in the past perfect tense using the auxiliary verb "had", which is correct.
3. "soundly slept": In English grammar, the position of an adverb of manner (like "soundly") in relation to a transitive/intransitive main verb typically comes after the verb, or the main verb comes after the auxiliary verb with proper placement. More specifically, adverbs of manner like "soundly" usually follow the intransitive verb ("slept soundly") or come between the auxiliary verb and the principal verb ("had soundly slept"). However, placing "soundly slept" directly after "he had" without standard positioning creates an awkward and grammatically incorrect word order in this context. The preferred order is "slept soundly" (e.g., "...showed that he had slept soundly the previous night").
4. "the previous night": This is an adverbial clause/phrase of time indicating when the action occurred, which is grammatically correct.

Therefore, the segment containing the grammatical error is "soundly slept".