Final answer:
Alzheimer's disease is the progressive neurological disorder that leads to cognitive decline. It is characterized by neuron death and a worsening of symptoms over time, distinguishing it from the other listed disorders. Hence, option (A) is correct.
Step-by-step explanation:
The progressive neurological disorder that leads to cognitive decline mentioned in the question is Alzheimer's disease (A). While disorders like bipolar disorder, obsessive-compulsive disorder (OCD), and attention-deficit/hyperactivity disorder (ADHD) affect mental health, they are not progressive neurological diseases that lead to a decline in cognitive function in the same way that Alzheimer's does.
Alzheimer's disease is characterized by the death of neurons and the subsequent worsening of symptoms over time which include memory loss, difficulties with language, and disorientation. Additionally, Alzheimer's along with Parkinson's disease are neurodegenerative diseases where the underlying cause seems to be related to proteins behaving improperly, such as beta-amyloid plaque buildup in Alzheimer's.