Consider the following statements.
Carboxylation of RuBP is catalysed by RuBisCO.
The first stable intermediate compound formed is phosphoglycerate.
18 ATP molecules are synthesised during dark cycle.
NADPH+`H^+` is used to reduce diphosphoglycerate
Choose the correct option.

Options

A. II,III and IV
B. I,III and IV
C. I,II and IV
D. I,II and III

Answer

To analyze the statements given in the question, we will evaluate each statement one by one:

1. **Carboxylation of RuBP is catalyzed by RuBisCO.**
- This statement is **correct**. The enzyme RuBisCO (Ribulose bisphosphate carboxylase/oxygenase) catalyzes the carboxylation reaction where carbon dioxide is fixed to ribulose bisphosphate (RuBP) to form 3-phosphoglycerate (3-PGA).

2. **The first stable intermediate compound formed is phosphoglycerate.**
- This statement is also **correct**. The product of the carboxylation of RuBP is 3-phosphoglycerate (PGA), which is indeed the first stable intermediate formed in the Calvin cycle.

3. **18 ATP molecules are synthesized during the dark cycle.**
- This statement is **incorrect**. During the dark reactions (Calvin cycle), ATP is consumed rather than synthesized. ATP is produced during the light-dependent reactions, not in the dark cycle.

4. **NADPH+H+ is used to reduce diphosphoglycerate.**
- This statement is **correct**. NADPH is used to reduce 1,3-bisphosphoglycerate (not diphosphoglycerate) to glyceraldehyde-3-phosphate (G3P) in the Calvin cycle.

Now, summarizing the correctness of each statement:
- Statement 1: Correct
- Statement 2: Correct
- Statement 3: Incorrect
- Statement 4: Correct

Based on this analysis, the correct options are statements 1, 2, and 4.

### Final Answer:
The correct statements are:
1. Carboxylation of RuBP is catalyzed by RuBisCO. (Correct)
2. The first stable intermediate compound formed is phosphoglycerate. (Correct)
3. 18 ATP molecules are synthesized during dark cycle. (Incorrect)
4. NADPH+H+ is used to reduce diphosphoglycerate. (Correct)

Thus, the correct option includes statements 1, 2, and 4.