Which of the following noise figure values is typical of a low-noise UHF preamplifier?
Why Noise figure measures how much a stage degrades the signal-to-noise ratio, expressed as a power ratio in dB, so it is always a positive number (a perfect noiseless amplifier would be 0 dB). Modern GaAsFET and HEMT devices at UHF give real-world noise figures well under 1 dB, so a value around half a dB is what a good low-noise preamp achieves. Anything much above 1 to 2 dB at UHF is considered mediocre for weak-signal work.
Watch out The choices given in dBm are absolute power levels, not ratios, so they cannot describe noise figure at all, and a negative dB noise figure would mean the amplifier improves the signal-to-noise ratio, which is physically impossible.
Noise figure is a ratio in dB, never dBm, and never negative. Good UHF preamp: a fraction of a dB.