Chlorophyll’s function is to absorb light — option 4.

What chlorophyll is. It is the green pigment held in the thylakoid membranes of the chloroplast. Its molecule has a porphyrin ring with a magnesium atom at the centre and a long hydrocarbon tail that anchors it in the membrane. The ring absorbs strongly in the blue and the red parts of the spectrum and reflects green — which is why leaves look green.

Where it acts in the process. Photosynthesis has two stages :

Why options 1 and 2 are so tempting. Both name things that genuinely happen in photosynthesis, and both depend on chlorophyll — but neither is what chlorophyll itself does. Water is split by the oxygen-evolving complex, a manganese-containing enzyme cluster at photosystem II, and CO₂ is reduced by the enzyme RuBisCO in the Calvin cycle. Chlorophyll’s single job is to capture the light energy that makes both possible. Recognising that a pigment is an absorber, not a catalyst, is the key to the question.

The overall equation :

\(6\text{CO}_{2}+6\text{H}_{2}\text{O}\xrightarrow{\ \text{light, chlorophyll}\ }\text{C}_{6}\text{H}_{12}\text{O}_{6}+6\text{O}_{2}\)

A useful detail. Chlorophyll absorbs green light least, so green light is the least effective for photosynthesis — a fact demonstrated by the classic experiments of Engelmann with a prism and aerobic bacteria. Accessory pigments such as carotenoids widen the range of wavelengths that can be used.

Hence, the answer is absorbing light.