The dark reaction takes place in the stroma of the chloroplast — option 3.

The structure of a chloroplast. It is a double-membrane organelle containing a system of flattened sacs and a fluid ground substance :

Why the two stages are separated. The light reaction needs the pigments held in a membrane, so that the energy of an absorbed photon can be passed along an electron-transport chain and used to pump protons; a membrane is essential to it. The dark reaction is ordinary enzyme chemistry — the fixing of CO₂ by RuBisCO and its reduction to sugar — and enzymes work in solution. So one stage is membrane-bound and the other is not.

What the dark reaction does. Carbon dioxide is attached to a five-carbon sugar, ribulose bisphosphate, and the product is reduced to carbohydrate using the ATP and NADPH made in the light reaction :

\(6\text{CO}_{2}+18\text{ATP}+12\text{NADPH}\rightarrow\text{C}_{6}\text{H}_{12}\text{O}_{6}+\ldots\)

A caution about the name. “Dark reaction” does not mean it happens at night or requires darkness — it means only that light is not directly involved. It stops soon after the light does, because it depends on the ATP and NADPH the light reaction supplies. For that reason it is better called the light-independent reaction or the Calvin cycle, after Melvin Calvin, who traced it with radioactive carbon-14 and received the Nobel Prize in 1961.

Hence, the answer is Stroma.