Which of the provided plots can represent the thermal energy...

The thermal energy $U$ generated in a resistor of resistance $R$ over a time interval $t$ due to an electric current $I$ is given by Joule's law of heating:

$U = I^2 R t$

For a given resistor and a constant time interval, $R$ and $t$ are constants. Therefore, the thermal energy is directly proportional to the square of the electric current:

$U \propto I^2$

This represents the equation of a parabola that opens upwards and is symmetric about the $U$-axis. Among the given plots, the solid curve $d$ correctly represents this parabolic relationship, as its slope increases with an increase in current $I$.

Thus, plot $d$ represents the thermal energy generated as a function of the electric current.

Answer: $d$