Introduction:
The light reaction, also known as the photochemical phase, is the first stage of photosynthesis. It occurs in the thylakoid membrane of the chloroplasts and involves the absorption of light energy to produce chemical energy in the form of ATP and NADPH. This process also releases oxygen as a byproduct.

Explanation:
The light reaction consists of several important steps that are directly related to its function. Let's examine each of these steps and determine which one is not related to the light reaction.

1. Light Absorption:
During the light reaction, chlorophyll pigments in the thylakoid membrane absorb light energy. This energy is used to excite electrons and initiate the process of photosynthesis.

2. Water Splitting:
One of the primary functions of the light reaction is to split water molecules into hydrogen ions (H+), electrons (e-), and oxygen (O2). This process is known as photolysis and occurs in the thylakoid lumen. The oxygen released during this step is a byproduct of photosynthesis.

3. Electron Transport:
After water splitting, the excited electrons from chlorophyll are passed through a series of electron carriers in the thylakoid membrane. This process is known as the electron transport chain. The energy released during electron transport is used to pump protons (H+) across the thylakoid membrane, creating a proton gradient.

4. ATP Synthesis:
As the protons flow back through ATP synthase, the enzyme uses their energy to convert ADP (adenosine diphosphate) into ATP (adenosine triphosphate). This process is called chemiosmosis and results in the production of ATP, which is a usable form of energy for the cell.

The correct answer:
From the given options, the one that is not related to the light reaction is option 'D', which is "Electron transport." Electron transport is an essential part of the light reaction and is responsible for the generation of ATP and NADPH. Therefore, it is directly related to the light reaction and cannot be the correct answer.