**Core Concept**
The underlying principle being tested involves the interpretation of radiological findings in the context of sellar and suprasellar pathology, particularly focusing on the **sella turcica** and its relation to **pituitary gland** function and surrounding neurological structures.

**Why the Correct Answer is Right**
Given that the X-rays show widening of the sella but neurological and endocrinological investigations are normal, the most likely diagnosis would be related to a condition that affects the sella turcica without immediately impacting pituitary function or surrounding neural structures. This scenario often points towards a **primary empty sella**, where the sella turcica is filled with cerebrospinal fluid, or other non-functioning, non-neoplastic conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it would typically present with hormonal imbalances or other specific symptoms related to the pituitary gland.
**Option B:** Incorrect as it would likely show abnormalities in endocrinological investigations.
**Option C:** Incorrect because it would have distinct neurological deficits.
**Option D:** Incorrect as it is not provided, implying the correct answer is one of the above or another condition not listed.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that a **primary empty sella** can be asymptomatic and discovered incidentally, with normal pituitary function, emphasizing the importance of correlating clinical findings with radiological results.

**Correct Answer:** D. Primary empty sella.