The first step in dark reaction of photosynthesis is

Options

A. Formation of `ATP`
B. Attachment of carbon dioxide to a pentose sugar
C. Excitement of an electron of chlorophyll by photon of light
D. Ionisation of water.

Answer

**Step-by-Step Solution:**

1. **Understanding Photosynthesis**: Photosynthesis consists of two main stages: light reactions and dark reactions (also known as the Calvin cycle). The light reactions occur in the thylakoid membranes, while the dark reactions take place in the stroma of the chloroplasts.

2. **Identifying the Dark Reaction**: The dark reaction is primarily responsible for the synthesis of glucose. It does not require light directly but relies on the products of the light reactions (ATP and NADPH).

3. **Recognizing the Key Enzyme**: The first step in the dark reaction involves the enzyme ribulose bisphosphate carboxylase/oxygenase (commonly known as Rubisco). This enzyme plays a crucial role in the carboxylation process.

4. **Carboxylation Process**: In this step, carbon dioxide (CO2) is fixed to a 5-carbon sugar, ribulose bisphosphate (RuBP). The reaction produces an unstable 6-carbon compound that quickly splits into two molecules of 3-phosphoglycerate (PGA), which is the first stable product of the dark reaction.

5. **Conclusion**: Therefore, the first step in the dark reaction of photosynthesis is the carboxylation of ribulose bisphosphate (RuBP) by carbon dioxide, facilitated by the enzyme Rubisco.

**Final Answer**: The first step in the dark reaction of photosynthesis is the carboxylation of ribulose bisphosphate (RuBP) by carbon dioxide, catalyzed by the enzyme Rubisco.

---