During exercise, muscle cells release the protein which can cause brite/beige adipocyte formation:
a) ghrelin
b) leptin
c) neuropeptide
d) irisin
e) britenin

Option "D".

Explanation: During exercise muscle cells release irisin protein is being released causing Beige adipocyte formation.

the secretion of Irisin leads to the improvement in muscle activity, bone mineral density, increasing the formation of brown like Beige adipocytes in the tissue.

Adipocytes are divided into two tissues namely white and brown. white tissues generally expand during excessive nutrition and brown adipocytes have mitochondria and burn the energy during exercises resulting into the break down of ATP  molecules increasing the body heat.  

Therefore, during exercise heat generated due to the formation of brown adipocyte tissue which are formed due to the release of irisin protein.