Q1 option C explain ...last word is steady state is reached.
For Q 5
Thermal radiation is directly proportional to 4th power of temperature.
For Q 6
In steady state no heat transfers so T must be constant
For Q 6 I got it wrong first
At the steady state the end near the furnace is hotter than the end far from the furnace
Same logic is applicable for Q1
As initial temp. =final Temp.
So change in internal energy is zero