The correct option is B that everyone in the office could hear him
An adverb clause is a group of words containing a subject and a verb,
and it functions as an adverb, by modifying an adjective, verb or an
adverb. Adverb clauses generally begin with a subordinate conjunction
(since, because, while, etc.)
From the options, "that everyone in the office could hear him" is
an adverb clause of cause, 'so...that' is a subordinate conjunction that shows cause, hence
option B is correct as it's the most suitable option.
Option A is incorrect because it is a phrase.
Option C is incorrect because it's an adverbial phrase, not a clause.
Option D is incorrect because the answer lies in B.