Ans Is C Infrared signals can be used for short range communication in a closed area using Line of sight propagation . infrared with range 300 ghz onwards lies in EHF and hence belongs to line of sight in give table
BAND range Propagation
VLF(very low frequency) 3-30 KHZ GROUND
LF(low) 30-300KHZ GROUND
MF(middle) 300KHZ--3MHZ SKY
HF(high) 3-30 MHZ SKY
VHF(very HF) 30-300 MHZ SKY & LINE OF SIGHT
UHF(Ultra Hf) 300MHZ-3GHZ LINE OF SIGHT
SHF(super HF) 3-30 GHZ LINE OF SIGHT
EHF (extremly high frequency) 30-300 GHZ LINE OF SIGHT