**Core Concept**
The question is testing the understanding of a toxin that acts on cyclic guanosine monophosphate (cGMP), a secondary messenger molecule involved in various physiological processes. cGMP is a key molecule in the regulation of smooth muscle tone, platelet aggregation, and neurotransmission.

**Why the Correct Answer is Right**
The toxin responsible for acting on cGMP is Oxytocinase is incorrect, Oxytocin is not the toxin, The correct toxin is Oxytocinase is incorrect, the toxin is actually **Botulinum toxin (Botulinum)** acts by inhibiting the release of acetylcholine, which in turn increases cGMP levels in smooth muscle cells, leading to muscle relaxation. This toxin is responsible for botulism, a rare but potentially fatal illness.

**Why Each Wrong Option is Incorrect**
* **Option A:** Oxytocin is incorrect because oxytocin is a hormone that stimulates uterine contractions and milk letdown during lactation, not a toxin that acts on cGMP.
* **Option B:** There is no toxin listed in option B.
* **Option C:** There is no toxin listed in option C.
* **Option D:** There is no toxin listed in option D.

**Clinical Pearl / High-Yield Fact**
Botulinum toxin is a potent neurotoxin that can cause muscle weakness, paralysis, and respiratory failure if not treated promptly.

**Correct Answer:** B. Botulinum toxin