The correct option is
C Presence of ferricyanide
The Hill reaction is defined as the photoreduction of an electron acceptor by the hydrogens of water, with the evolution of oxygen. In vivo, or in the organism the final electron acceptor is NADP+. In plants, the evolution of oxygen during the light-dependent steps of photosynthesis is referred to as Hill reaction as this was proposed and proven by the British biochemist Robin Hill. He demonstrated that isolated chloroplasts would
make oxygen (O
2) but cannot fix CO2. His experiments led him to the fact that isolated chloroplasts from plants can release oxygen when they are illuminated by sunlight in the presence of a suitable electron receptor such as ferricyanide
So, the correct answer is 'Presence of ferricyanide'