Solution

We need to evaluate the truthfulness of both the Assertion (A) and the Reason (R), and then check if R is the correct explanation for A.

• Assertion (A): "Goitre is a common disease in mountainous regions." This is a correct statement.
  
• Reason (R): "The diet of the people in mountains lacks Iodine content." This is also a correct statement. A lack of iodine is the primary cause of goitre.
  

Conclusion:

The Reason (R) directly and correctly explains the Assertion (A). The lack of iodine in the diet of people in mountainous regions is the scientific reason for the high prevalence of goitre there. Therefore, both statements are correct, and R is the correct explanation of A.