Final answer:
The plasma membrane is selectively permeable and fluid, allowing certain substances to pass while maintaining cellular homeostasis. It is composed of lipids, proteins, and carbohydrates in varying proportions specific to cell type.
Step-by-step explanation:
The plasma membrane is integral to cellular function, offering a dynamic barrier between the cell's internal environment and the extracellular space. When considering which of the following are true of the plasma membrane, statement B) It is selectively permeable is correct. This means the plasma membrane allows certain substances to pass while blocking others, maintaining homeostasis by controlling the internal composition of the cell. Additionally, statement C) It is fluid is also true. The term "fluid" here means that the components of the membrane such as lipids and proteins can move around laterally, providing a flexible structure for cellular processes.
On the other hand, statement A) It is fully permeable is incorrect because if the plasma membrane allowed all substances to freely pass through, the cell would not be able to sustain its internal environment.
Regarding statement D) It is composed mainly of carbohydrates and proteins, this is partially true as the membrane indeed has proteins but it's not mainly composed of carbohydrates. In fact, the principal components of a plasma membrane are lipids (about 40%), proteins (about 50%), and a smaller percentage of carbohydrates (about 10%) that are primarily found on the membrane's exterior, contributing to cell recognition and signaling.