Assertion (A) : More and more children in metro cities of India suffer from allergies and asthma due to sensitivity to the environment.

Reason (R) : Modern day lifestyle and a protected environment in early life has resulted in lowering the immunity.

The correct option is: Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).

Step-by-step Explanation:

1. Analysis of Assertion (A):
Assertion (A) states: "More and more children in metro cities of India suffer from allergies and asthma due to sensitivity to the environment."
This statement is factually true. Industrialization, heavy vehicular emissions, dust, and general pollution in metropolitan cities expose children to a high concentration of allergens. Consequently, there has been a documented rise in respiratory issues like asthma and allergic reactions among children living in urban environments.

2. Analysis of Reason (R):
Reason (R) states: "Modern day lifestyle and a protected environment in early life has resulted in lowering the immunity."
This statement is also true and is explained by a well-established immunological concept known as the "Hygiene Hypothesis." In modern lifestyles, children are often raised in highly sanitized, clean, and protected environments. Due to this lack of early exposure to diverse microbes, dirt, and natural antigens, their immune systems do not get trained properly. As a result, the immune system becomes hypersensitive and begins reacting to harmless environmental particles (like pollen or dust), thereby lowering overall immunological tolerance.

3. Evaluating the Link between Assertion (A) and Reason (R):
Because children in metro cities are raised in highly protected, sanitised indoor environments (modern lifestyle) and then exposed to polluted city environments, their untrained immune systems react aggressively. Thus, the lower immunity and hypersensitivity caused by a protected early life (Reason R) directly explain why more children in metropolitan areas suffer from allergies and asthma (Assertion A).

Therefore, both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).