A black body is at a temperature 300 K. It emits energy at a rate, which is proportional to

From Stefan's law the total radiant energy (E) emitted per second per unit surface area of a black body is proportional to the fourth power of the absolute temperture (T) of the body

i.e., E = ${\mathrm{\sigma T}}^4$

where $\mathrm{\sigma }$is Stefan's constant

Hence, E directly proportional to ${\left(300\right)}^4$