The process which makes major difference between ${C_3}$ and ${C_4}$ plants is
A. photorespiration
B. respiration
C. glycolysis
D. calvin cycle

Hint: ${C_3}$ plants are defined as the plants that exhibit the ${C_3}$ pathway. These plants use the Calvin cycle in the dark reaction of photosynthesis. On the other hand, ${C_4}$ plants are defined as the plants that use the ${C_4}$ pathway or Hatch-slack pathway during the dark reaction.

Complete answer:
${C_3}$ Plants are those where the initial product is 3-phosphoglycerate with 3 carbon atoms. These plants are also known as temperate plants. These plants reduce into carbon dioxide directly in the chloroplast.
${C_4}$ plants are plants which cycle carbon dioxide to 4-carbon sugar compounds in order to enter the ${C_3}$ or the Calvin cycle. The ${C_4}$ plants are very productive in climatic conditions that are hot and dry and produce a lot of energy.

Photorespiration -A process in plant metabolism where enzyme RuBisCO oxygenates RuBP, causing some of the energy produced by photosynthesis to be wasted.
Where the ${O_2}$ enters the Calvin cycle instead of $C{O_2}$ this results in reduced rate of photosynthesis.
Calvin cycle -Involves Carbon dioxide fixation in the presence of ATP and NADPH and its conversion to glucose, through a series of reactions
Regular mode of carbon dioxide fixation in a number of tropical plants. First product is 4-carbon compound, hence, known as ${C_4}$

However the Photorespiration is present in ${C_3}$ but almost absent in ${C_4}$ plants.

Hence, the correct answer is option (A).

Note: Photorespiration is a process that wastes energy and decreases carbohydrate synthesis. Therefore, in order to prevent photorespiration, there are three different ways that dark reactions occur in plants to prevent the meeting of oxygen with Rubisco. Hence, depending on the way that dark reaction takes place, there are 3 types of plants; namely, ${C_3}$ plants, ${C_4}$ plants, and CAM plants.