The process which makes major difference between \[\mathop C\nolimits_3 \] and \[\mathop C\nolimits_4 \] plants is
(A) Respiration
(B) Glycolysis
(C) Calvin cycle
(D) Photorespiration

Hint: Melvin Calvin uses radioactive \[\mathop {14}\nolimits_C \] isotope in algal photosynthesis studies. This led to discovery that the first \[\mathop {CO}\nolimits_2 \] fixation product was a three-carbon organic acid so he named this cycle as \[\mathop C\nolimits_3 \] cycle. \[\mathop C\nolimits_4 \] is also known as the Hatch – slack pathway. This cycle is performed by plants that are adapted to dry tropical regions as that of sugarcane.

Complete answer:
\[\mathop C\nolimits_4 \] plants can tolerate saline conditions due to abnormal due to abundant occurrence of organic acids in them which lowers their water potential more than that of soil . They can tolerate high ranges of light intensities , high salinity ranges due to a lack of process that is known as photorespiration. This process is exhibited by \[\mathop C\nolimits_3 \]
The \[\mathop C\nolimits_4 \] plants have mechanisms that increase concentration of \[\mathop {CO}\nolimits_2 \] at the enzyme site and due to this they lack photorespiration.
Now let us match this process with the options given :-
Respiration:- this is a common process that occurs in both types of processes . Hence, this option is not correct.
Glycolysis:- it is the first step of respiration which involves conversion of glucose into pyruvate in both the plants . Thus this option is not correct.
Calvin cycle :- \[\mathop C\nolimits_3 \] cycle is also known as Calvin cycle . It is not a process. Thus, this option is not correct.
Photorespiration :- The \[\mathop C\nolimits_4 \] plants have mechanism that increase concentration of \[\mathop {CO}\nolimits_2 \] at the enzyme site and due to this they lack photorespiration but \[\mathop C\nolimits_3 \] plants possess it . Thus, this option is correct.

Our required answer is d that is photorespiration.

Note: Photorespiration is a process which involves loss of fixed carbon as \[\mathop {CO}\nolimits_2 \] in plants in the presence of light.It is initiated in chloroplast . This process does not produce ATP and NADPH and is a wasteful process. This process occurs usually when there is high concentration of oxygen.