**Core Concept**
The question is testing the understanding of age-related changes in lung function. With increasing age, there are several physiological changes that occur in the lungs, affecting their structure and function. These changes can impact respiratory health and disease susceptibility.

**Why the Correct Answer is Right**
One of the significant changes in lung function with age is the decline in lung elasticity and the ability to expand and fill with air. This is due to the loss of elastic fibers in the lung parenchyma, particularly in the alveolar walls. As a result, older adults experience a decrease in vital capacity and forced expiratory volume, making it more difficult to breathe. This decline is a gradual process, typically starting in the fourth decade of life and accelerating with advancing age.

**Why Each Wrong Option is Incorrect**
**Option A:** This option does not accurately describe the change in lung function with age. While it is true that older adults experience a decrease in lung function, this option does not provide a specific and accurate description of the change.

**Option B:** This option is incorrect because the change in lung function with age is not limited to the diaphragm. While the diaphragm does become less efficient with age, the primary change is in the lung parenchyma itself.

**Option C:** This option is incorrect because the change in lung function with age is not primarily due to an increase in airway resistance. While airway resistance does increase with age, this is not the primary factor contributing to the decline in lung function.

**Clinical Pearl / High-Yield Fact**
As a general rule, lung function declines with age, and this decline accelerates in the sixth and seventh decades of life. It is essential to consider age-related changes in lung function when evaluating respiratory symptoms and disease in older adults.

**Correct Answer:** C.