**Core Concept**
Biofilms are complex communities of microorganisms that adhere to a surface, embedded in a protective extracellular matrix. This matrix, composed of polysaccharides, proteins, and DNA, provides a shield against environmental stress, antimicrobial agents, and the host's immune system.

**Why the Correct Answer is Right**
The formation of biofilms involves the coordinated action of several factors, including surface attachment, cell-to-cell signaling, and the production of the extracellular matrix. The matrix is primarily composed of polysaccharides, such as alginate, which is produced by Pseudomonas aeruginosa. The extracellular matrix also contains proteins, such as flagellin, and DNA, which contribute to its stability and resistance to antimicrobial agents.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because biofilms do not necessarily have a single, dominant species. They are often composed of multiple species that interact and cooperate to form a complex community.
**Option B:** This option is incorrect because biofilms are not simply clusters of microorganisms. They are organized communities with a distinct structure and function, including the production of a protective extracellular matrix.
**Option C:** This option is incorrect because biofilms are not formed solely by the action of enzymes. While enzymes, such as alginate lyase, may play a role in biofilm formation, they are not the primary mechanism.

**Clinical Pearl / High-Yield Fact**
Biofilms are responsible for many chronic infections, including those of the respiratory tract, urinary tract, and prosthetic joints. They are particularly resistant to antimicrobial therapy, making treatment challenging.

**Correct Answer: D. They are resistant to antimicrobial agents and the host's immune system due to their protective extracellular matrix.**