What instrument is used to locate and measure interfering narrowband RF signals in an IEEE 802.11 WLAN?

The correct choice is the spectrum analyzer, which is specifically designed to locate and measure radio frequency (RF) signals. In the context of an IEEE 802.11 WLAN (Wireless Local Area Network), a spectrum analyzer can visualize the RF spectrum. This allows a technician to see the different frequencies being utilized and identify any interfering narrowband signals that may disrupt WLAN performance.

Spectrum analyzers provide essential information such as signal amplitude versus frequency, enabling technicians to diagnose problems with signal interference effectively. By analyzing the spectrum, one can identify the presence of unauthorized devices or other RF sources that could cause degradation of the WLAN signal quality.

The other instruments listed have different functions that do not pertain directly to identifying or measuring RF signals: a multimeter is used for measuring electrical voltage, current, and resistance; a signal booster amplifies the strength of an existing signal but does not analyze it; and a network cable tester verifies the integrity of network cables rather than RF signals. Each of these tools serves a valuable purpose in their respective areas, but only a spectrum analyzer is tailored for dealing with RF signal detection and measurement in WLAN environments.