**Core Concept:** Opium is a natural product derived from the dried latex of the opium poppy plant (Papaver somniferum). It contains various alkaloids, including morphine, codeine, and thebaine. The question is focusing on the presence of an antitussive agent in opium with distinct effects on pain and addiction.

**Why the Correct Answer is Right:** The correct answer is "Codeine" (Option B). Codeine is an opioid alkaloid present in opium which is an effective antitussive (cough suppressant) without the significant analgesic (pain relieving) or addicting properties of morphine and thebaine. Codeine is a prodrug, meaning it is not active in its native form but requires microbial or enzymatic conversion to its active form, morphine, for its effects.

**Why Each Wrong Option is Incorrect:**

A. Morphine (Option A) is a potent opioid alkaloid found in opium. It is a potent analgesic, has significant sedative and respiratory depression effects, and is highly addictive, unlike codeine.

C. Thebaine (Option C) is another opioid alkaloid found in opium. Thebaine is a precursor to morphine and codeine, but does not possess analgesic or antitussive properties on its own.

D. Opium (Option D) is the raw material from which codeine, morphine, and thebaine are derived. It is not a specific agent with distinct effects, as it contains multiple alkaloids.

**Clinical Pearl:** Codeine is commonly used as a cough suppressant in over-the-counter cough syrups due to its efficacy and reduced side effects compared to morphine. However, it is important to note that codeine can still cause side effects such as nausea, vomiting, and drowsiness.

**Correct Answer:** Codeine (Option B) is the correct choice because it is an antitussive agent present in opium, without significant analgesic or addictive properties as found in morphine and thebaine. Codeine's antitussive effect results from its conversion to morphine, which binds to opioid receptors, inhibiting cough reflex.