Let Q be the temperature at a distance x from hot end of bar. Let Q is the temperature of hot end. The heat flow rate is given by
dQ
dt
=
kA(θ1−θ)
x
⇒θ1−θ=
x
kA
dQ
dt
⇒θ=θ1−
x
kA
dQ
dt
Thus, the graph of Q versus x is a straight line with a positive intercept and a negative slope. The above equation can be graphically represented by option (a).