Step 1: Understanding the Concept:
This is a sentence completion question that relies on understanding the contrast between two related biological processes: inhalation and exhalation.
Step 2: Detailed Explanation:
The sentence starts with the key phrase "In contrast to," which sets up an opposition. We are told that inhalation requires "substantial muscular activity." Therefore, exhalation must be a process that does \textit{not} require substantial muscular activity. We need to find the word that best describes this lack of active effort.
Let's evaluate the options:
(A) slow: Exhalation can be fast or slow; this word doesn't capture the contrast with muscular activity.
(B) passive: A "passive" process is one that does not require active energy or effort. This is the direct opposite of a process requiring "substantial muscular activity." This is a perfect fit. (In biology, normal exhalation is indeed a passive process, relying on the elastic recoil of the lungs and chest wall).
(C) precise: Both inhalation and exhalation can be precise; this doesn't contrast with muscular effort.
(D) complex: Both processes are complex in their own way. This is not the point of contrast.
(E) conscious: While we can consciously control our breathing, normal exhalation is typically an unconscious process, but "passive" is a much better antonym for "active" (as implied by muscular activity).
Step 3: Final Answer:
The word "passive" provides the most accurate and direct contrast to the "substantial muscular activity" of inhalation.