The term, "floating respiration" is used when the respiratory substrate is

Concept: All living things need strength to support the vital processes of life. This energy is obtained from the food we consume. However, to get the energy stored in food, the nutrients present must be broken down into simple substances.
Light sugars, amino acids, and fatty acids are absorbed by cells where they play different roles. One such role is their use as fuel during respiration.
Respiration involves the breakdown of nutrients, especially glucose, in order to obtain energy stored in its chemical bonds. This energy is then stored in ATP form. When carbohydrates or fats behave as substrates, it is called floating respiration.

Hence, the correct answer is option C.