Environmental microbiology — A dense bacterial population embedded in a tangled web of extracellular fibers and firmly attached to a surface is best described as:

Introduction / Context:
In aquatic systems, pipelines, medical devices, and natural habitats, many microbes live not as free cells but in surface-attached communities called biofilms. These communities are encased in extracellular polymeric substances (EPS), creating a protective matrix with distinct physiology and resilience.

Given Data / Assumptions:

• Description mentions a tangled web of fibers and surface attachment.
• We distinguish operational lab terms (e.g., membrane filtration) from ecological structures (biofilm).

Concept / Approach:
Biofilms result from adhesion of pioneer cells, EPS production, and subsequent colonization, forming a hydrated matrix that traps nutrients and cells. This differentiates them from process terms like coagulation (chemical destabilization/agglomeration) or equipment like biodiscs (rotating biological contactors where biofilms grow, but the biofilm is the structure itself, not the device).

Step-by-Step Solution:

Verification / Alternative check:
Microscopy of pipe scales and dental plaque shows classic biofilm morphology with EPS strands and embedded microcolonies.

Why Other Options Are Wrong:

• Coagulation: particle destabilization in water treatment, not surface-attached EPS matrices.
• Biodisc: equipment that supports biofilm growth; not the descriptive term.
• Membrane filter technique: lab method for enumerating bacteria; unrelated to surface matrices.

Common Pitfalls:
Calling any aggregate a biofilm; true biofilms are surface-attached and EPS-encased.

Final Answer:
a biofilm