The relation among minimum detectable signal (MDS), IF bandwidth (BW) and noise figure (NF) of a spectrum analyzer is 

• Minimum detectable signal (MDS) refers to the smallest signal power that a receiver or measurement system can reliably detect in the presence of noise. 
• In the context of a spectrum analyze, MDS represents the weakest signal that can be distinguished from the background noise.

MDS = 10\(\times\)log₁₀ (\(KT\over 1mW\))+ Noise figure + 10\(\times\)log₁₀ (Bandwidth)

MDS = minimum detectable signal in dBm.

K= Boltzmann's constant (-228\(\frac {dW} { V}\). Hz)

Noise figure in db.

T = Temperature in kelvin.

Bandwidth in Hz.

Here, option 3 is correct.