A new born baby has the cold resisting device due to -

The correct option is A Brown fat
Brown fat, also called brown adipose tissue, is a special type of body fat that is turned on (activated) when you get cold. Brown fat produces heat to help maintain your body temperature in cold conditions. Brown fat contains many mitochondria.
The adipose tissue is a central metabolic organ in the regulation of whole-body energy homeostasis. It includes white adipose fat and brown adipose fat.
Reticular connective tissues are found in the kidney, the spleen, lymph nodes, and bone marrow. Their function is to form a stroma and provide structural support to lymphoid organs.

So, the correct answer is- option a.