The difference between received power level and minimum required signal level at the input to the receiver✓الإجابة الصحيحة
Transmit power minus receiver sensitivity
Receiver sensitivity plus 3 dB
الشرح
Link margin is the headroom the link budget leaves over: how far the received signal level sits above the minimum the receiver needs. Transmit power minus receiver sensitivity ignores path loss and antenna gains, so it is not the margin actually available.