The first hypothesis that oxygen released during photosynthesis is derived from water was proposed by
A. Engelmann
B. Priestley
C. Van Niel
D. Blackman

Hint: He studied photosynthesis in purple sulfur bacteria.

Complete answer:Van Niel studied photosynthesis on purple Sulfur bacteria, where he found that oxygen released during photosynthesis is derived from water.
According to Van Niel’s theory the equation of photosynthesis is-
6$C0_2$ + 12$H_2$0 → $C_6H_12O_6$ + 6$H_2$O +6$O_2$

Additional information:Van Niel while studying purple sulfur bacteria first described the importance of light in photosynthesis.
1. The purple sulfur bacteria synthesize glucose from $CO_2$ like green plants and for this process light plays an important role.
2. No oxygen was produced during this process, rather elemental sulfur was produced.
3. Van Niel explained that the action of light caused decomposition of H2S into hydrogen and sulfur atoms.
4. After a dark reaction, the hydrogen atoms were used to reduce $CO_2$ to carbohydrates.
5. Van Niel described a parallel visionary with the green plants. He described that in green plants the light causes water to break up into hydrogen and oxygen.
6. In the dark reaction, hydrogen atoms are used to reduce $CO_2$.

So, the answer is C. Van Niel

Note:Engelmann produced the first action spectrum for photosynthetic oxygen evolution.
Priestley described that the air which is ‘noxious’ by breathing or burning of a candle can be restored by green plants. He isolated the gas later as Oxygen.
Blackman described the phenomenon of limiting factor on photosynthesis.