<p>The correct answer is <strong>(B) exact</strong>.</p>
<p>In this sentence, we need to use an adjective to describe the state of the humidity and temperature in the laboratory. The word "exact" is the correct choice because it is an adjective that means precise or accurate.</p>
<p>The word "exactly" (option A) is an adverb, which is used to modify a verb, adjective, or other adverb. In this sentence, we need an adjective to describe the state of the humidity and temperature, not an adverb.</p>
<p>The word "exactness" (option C) is a noun, which refers to the quality of being exact or precise. However, in this sentence, we need an adjective to describe the state of the humidity and temperature, not a noun.</p>
<p>The word "exacted" (option D) is a past tense verb, which means to demand or obtain something from someone. It does not fit the context of the sentence, which requires an adjective to describe the state of the humidity and temperature.</p>