**Core Concept**
Struma ovarii is a rare form of ovarian teratoma that is composed of thyroid tissue. It is a type of mature cystic teratoma that contains thyroid tissue, which may be functioning or non-functioning. The presence of thyroid tissue in the ovary can lead to clinical manifestations such as hyperthyroidism, hypothyroidism, or euthyroidism.

**Why the Correct Answer is Right**
The correct answer is because struma ovarii is a specific type of ovarian teratoma that is characterized by the presence of thyroid tissue. This tissue can be in the form of thyroid follicles, which are lined by follicular cells that produce thyroid hormones. The thyroid tissue in struma ovarii can be functioning, meaning it produces thyroid hormones, or non-functioning, meaning it does not produce thyroid hormones. The presence of thyroid tissue in the ovary can lead to a variety of clinical manifestations, including hyperthyroidism, hypothyroidism, or euthyroidism.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because struma ovarii is not composed of adrenal tissue. While some ovarian teratomas may contain adrenal tissue, struma ovarii is specifically characterized by the presence of thyroid tissue.

**Option B:** This option is incorrect because struma ovarii is not composed of pancreatic tissue. While some ovarian teratomas may contain pancreatic tissue, struma ovarii is specifically characterized by the presence of thyroid tissue.

**Option C:** This option is incorrect because struma ovarii is not composed of intestinal tissue. While some ovarian teratomas may contain intestinal tissue, struma ovarii is specifically characterized by the presence of thyroid tissue.

**Clinical Pearl / High-Yield Fact**
Struma ovarii is a rare form of ovarian teratoma that can present with a variety of clinical manifestations, including hyperthyroidism, hypothyroidism, or euthyroidism. It is essential to consider struma ovarii in the differential diagnosis of ovarian masses, especially in patients with thyroid abnormalities.

**Correct Answer:** D.