**Core Concept**
The underlying diagnosis is a condition that presents with recurrent, self-limiting ulcerations of the oral mucosa, often associated with viral infections. This condition involves the reactivation of a latent virus that causes the characteristic oral lesions.

**Why the Correct Answer is Right**
The most likely diagnosis is Hand-Foot-and-Mouth disease (HFMD) due to coxsackievirus A, which is a common cause of oral ulcers in children. The virus infects the mucosal epithelium, leading to the formation of small, superficial ulcers that heal spontaneously within a few days. The irritability and withdrawal from the daycare center may be secondary to the pain and discomfort caused by the oral ulcers.

**Why Each Wrong Option is Incorrect**
**Option A:** Erythema multiforme (EM) is a more severe condition that presents with target lesions and can be associated with various triggers, including infections and medications. However, EM typically involves the skin and mucous membranes, and the oral ulcers are usually more extensive and deeper than those seen in HFMD.

**Option B:** Aphthous stomatitis, also known as canker sores, are typically solitary and more painful than the multiple ulcers seen in HFMD. They are also more common in adults and are often associated with stress, trauma, or underlying systemic conditions.

**Option C:** Oral hairy leukoplakia is a condition caused by Epstein-Barr virus (EBV) and is typically seen in immunocompromised patients, such as those with HIV/AIDS. It presents with white, hairy patches on the lateral aspects of the tongue, rather than multiple small ulcers.

**Option D:** Herpangina is another viral infection caused by coxsackieviruses and enteroviruses, which presents with painful ulcers on the tonsillar pillars and soft palate. While it can occur in children, it is less common than HFMD and typically involves the posterior oral cavity.

**Clinical Pearl / High-Yield Fact**
Recurrent oral ulcers in children should raise suspicion for coxsackievirus infections, such as HFMD or herpangina. A high index of suspicion and clinical examination can lead to early diagnosis and treatment.

**Correct Answer:** C.