The reason for choosing the adverb "apparently" to fill in the blank is that it is used to indicate that something seems to be the case based on available evidence or information. In the sentence, it suggests that Williams had blacked out based on the information or evidence at hand. The other adverbs do not convey the same meaning or fit the context of the sentence as effectively:

1. "Appear" is a verb and does not work as an adverb in this context.
2. "True" is an adjective and does not work as an adverb in this context.
3. "Somewhat" is an adverb, but it does not convey the intended meaning of seeming to be the case based on evidence.