Which of the following occurs during the light-dependent reactions of plants?
a. electron transport
b. the splitting of water
c. a hydrogen ion gradient is established to produce at
d. all the above

The correct answer will be option-D.

Explanation:

Photosynthesis proceeds in two stages: light-dependent and light-independent reaction.

The light-dependent begins when the chlorophyll pigment of the photosystem absorbs light energy.  This absorption causes the excitation of the electrons provided by the chlorophyll molecule.  

The excited electrons accepted by electron acceptor molecule from reaction center and a chain of electron flow begin called electron transport chain.

This transport of electrons creates a scarcity of electron in photosystem which causes splitting of the water molecule as a result of which oxygen gas is released.  

Thus, option-D is the correct answer.