**Core Concept**
Loeffler's syndrome is a condition characterized by the presence of eosinophils in the sputum and radiographic evidence of pulmonary infiltrates or nodules, often associated with parasitic infections or allergic reactions. It is a type of hypersensitivity reaction that occurs in response to an inhaled antigen.

**Why the Correct Answer is Right**
Loeffler's syndrome is typically seen in parasitic infections that involve the lungs, such as Toxocara, Strongyloides, and Ascaris. These parasites can cause an allergic reaction in the lungs, leading to the characteristic eosinophilia and pulmonary infiltrates. Giardia, on the other hand, is a gastrointestinal parasite that primarily causes infection in the small intestine and does not typically cause Loeffler's syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** Toxocara is a well-known cause of Loeffler's syndrome, as it can cause an allergic reaction in the lungs when larvae are inhaled.
**Option B:** Strongyloides is also a common cause of Loeffler's syndrome, as it can cause an allergic reaction in the lungs when larvae are inhaled.
**Option C:** Ascaris can also cause Loeffler's syndrome, as it can cause an allergic reaction in the lungs when larvae are inhaled.

**Clinical Pearl / High-Yield Fact**
It's worth noting that Loeffler's syndrome can also be caused by other parasites, such as hookworms and roundworms, and can be seen in other conditions such as allergic bronchopulmonary aspergillosis.

**Correct Answer:**
✓ Correct Answer: D. Giardia