Final answer:
Astrocytoma's (Option A) account for 50% of all primary brain tumors, making them the most common type among primary brain tumors. They are a type of cancer starting in astrocytes, supportive brain cells. Other options listed are not as common; pituitary adenomas can cause issues such as tunnel vision if affecting the optic chiasm.
Step-by-step explanation:
The tumors accounting for 50% of all primary brain tumors are astrocytoma's (Option A). Astrocytoma's are a type of cancer of the brain that start in cells called astrocytes, which are supportive tissue of the brain. Unlike the other options provided, astrocytomas are indeed the most common kind of primary brain tumor.
Pituitary adenomas (Option C), while not the most common primary brain tumor, can have significant health effects as well. For instance, if the pituitary gland develops a tumor, such as those seen in Cushing's syndrome, it can press against the optic chiasm, causing visual disturbances like tunnel vision—where only central vision is perceived.
It's important to note that schwannomas (Option B) and bone tumors (Option D) also occur within the brain, but they do not account for the majority of primary brain tumors as astrocytoma's do.