Filters: which statement best defines the behavior of a low-pass filter in signal processing?

Introduction / Context:
Filters shape spectra. A low-pass filter (LPF) preserves low-frequency content while attenuating high-frequency components. LPFs are fundamental in audio processing, anti-aliasing before digitization, smoothing sensor outputs, and conditioning power supplies.

Given Data / Assumptions:

• We consider an idealized description of a low-pass filter.
• “Passes” means minimal attenuation in the passband; “reduces” means attenuation in the stopband.
• Cutoff frequency marks the transition band between pass and stop regions.

Concept / Approach:
An LPF transfer function has a magnitude close to 1 (0 dB) at low frequencies and decreases as frequency rises beyond the cutoff (e.g., -20 dB/decade for a first-order RC filter). The phase also changes, typically approaching -90 degrees for a simple RC as frequency moves far beyond the cutoff.

Step-by-Step Solution:
1) Identify desired filter type: low-pass.2) Recall its function: preserve low frequencies; attenuate highs.3) Map options: Option A states the canonical definition.4) Select option A.

Verification / Alternative check:
Examine a first-order RC low-pass where |H(jω)| = 1 / sqrt(1 + (ωRC)^2). As ω increases, magnitude decreases—confirming high-frequency attenuation.

Why Other Options Are Wrong:
Option B defines a high-pass. Option C is incorrect; filters affect AC signals by design. Option D describes a fixed attenuator, not a selective filter.

Common Pitfalls:
Confusing cutoff frequency’s exact definition (e.g., -3 dB point) or mixing low-pass with high-pass responses.

Final Answer:
Passes lower-frequency signals and reduces higher-frequency signals.