**Core Concept**
The question is testing the student's knowledge of thyroiditis, specifically a condition characterized by inflammation of the thyroid gland without associated pain. This condition is often associated with a transient period of hyperthyroidism followed by hypothyroidism.

**Why the Correct Answer is Right**
The condition in question, known as "Painless Thyroiditis," is most commonly caused by a viral infection, such as Coxsackie B virus, which triggers an autoimmune response leading to inflammation of the thyroid gland. This results in the release of pre-formed thyroid hormones into the bloodstream, causing a transient period of hyperthyroidism. As the thyroid gland becomes depleted of these hormones, the patient may then develop hypothyroidism.

**Why Each Wrong Option is Incorrect**
**Option A:** De Quervain's thyroiditis is a different condition, characterized by a painful thyroiditis, often associated with a viral infection.
**Option B:** Hashimoto's thyroiditis is an autoimmune condition leading to hypothyroidism, but it does not typically present with a painless thyroiditis.

**Clinical Pearl / High-Yield Fact**
It's essential to distinguish between different types of thyroiditis, as the management and prognosis can vary significantly. Remember that painless thyroiditis often resolves spontaneously, but may require suppressive therapy to prevent long-term hypothyroidism.

**Correct Answer: B. Hashimoto's thyroiditis.**