Ans.
In an ecosystem, energy continuously gets in mainly from the sun, and transfers through the food chain. Photosynthetic plants transfer energy to herbivores and then, energy is transferred to carnivores from herbivores.
Organisms perform cellular respiration to obtain energy for daily-life functions through the breakdown of food. During energy transfer, some energy is lost to the environment, when organisms produce heat due to breakdown of bonds as a byproduct of cellular respiration.
Thus, the correct answer is option B). 'heat.'