Exposure to sunlight helps a person improve health mainly because which of the following processes occurs in the body?

Introduction / Context:
This general science question asks why exposure to sunlight is considered beneficial for health. While sunshine has multiple effects, competitive exams focus on the well known role of sunlight, particularly ultraviolet rays, in the synthesis of Vitamin D in the human body. Knowing this mechanism is important for basic biology and health related general knowledge.

Given Data / Assumptions:

• The question is about the main health benefit of sunlight exposure.
• Options mention infrared light, general resistance, tanning and ultraviolet rays converting skin oil into Vitamin D.
• We assume normal, controlled exposure, not excessive sunlight causing sunburn.

Concept / Approach:
The most important health related effect of moderate sunlight exposure is its role in Vitamin D synthesis. Ultraviolet B (UVB) rays from the sun react with a cholesterol related substance in the skin, converting it into a precursor that the body further processes into active Vitamin D. Vitamin D is essential for healthy bones, calcium metabolism and overall well being. The other options either describe cosmetic effects or oversimplify immunity, and do not capture this established scientific mechanism.

Step-by-Step Solution:
Evaluate option A: “The infrared light kills bacteria in the body.” Infrared light mainly provides heat. It is not a targeted antibacterial treatment inside the body, so this explanation is not accepted as the primary health benefit of sunlight. Evaluate option B: “Resistance power increases for all diseases instantly.” While Vitamin D may support immune function over time, sunlight does not magically or instantly increase resistance to all diseases. This statement is vague and exaggerated. Evaluate option C: “The pigment cells in the skin get stimulated and produce a healthy tan.” Tanning is indeed caused by melanin production in response to UV radiation, but a tan itself is not the main scientifically cited benefit and can even be a sign of skin damage if excessive. Evaluate option D: “The ultraviolet rays convert certain skin oils into Vitamin D.” This correctly describes the biochemical process where UV rays act on substances in the skin to start Vitamin D production. This is the recognised health benefit highlighted in textbooks. Therefore, option D best matches the medically accepted reason why sunlight helps improve health.

Verification / Alternative check:
Biology and health science texts state that when ultraviolet B rays from sunlight strike the skin, they convert 7 dehydrocholesterol into cholecalciferol, a form of Vitamin D3. This is then further processed by the liver and kidneys into active Vitamin D, which helps regulate calcium and phosphate levels for bone health. This chain of events directly supports option D. The other options either mischaracterise the effect of different parts of the spectrum or confuse cosmetic changes with health benefits.

Why Other Options Are Wrong:
Option A is wrong because infrared radiation mainly causes heating and does not selectively kill internal bacteria. Option B is wrong as it overstates the effect, suggesting instant general immunity, which is not accurate. Option C describes tanning, which is a visible skin change but is not the primary health advantage of sunlight and may be harmful if excessive. Only option D provides the correct, specific mechanism that links sunlight to improved health through Vitamin D formation.

Common Pitfalls:
Some candidates choose the answer about tanning because it sounds familiar, but the question asks about improving health, not just changing appearance. Others may mistakenly believe that any part of sunlight directly kills bacteria inside the body. To avoid these mistakes, remember the key role of Vitamin D in bone health and the fact that its production is triggered by ultraviolet light acting on the skin. Linking sunlight to Vitamin D is the most direct and widely accepted scientific explanation.

Final Answer:
Exposure to sunlight helps health mainly because ultraviolet rays convert certain skin oils into Vitamin D, so option D is correct.