The pairs of compounds which cannot exist together in aquous solution are:
(I)`NaH_(2)PO_(4) and Na_(2)HCO_(3)` (II)`Na_(2)CO_(3) and NaOH`
(III)`NaOH and NaH_(2)PO_(4)` (IV)`NaHCO_(3) and NaOH`

Options

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

Answer

To determine which pairs of compounds cannot exist together in aqueous solution, we need to analyze the reactions that occur between the compounds in each pair.

### Step-by-Step Solution:

1. **Identify the Compounds in Each Pair:**
- Pair I: `NaH₂PO₄` (sodium dihydrogen phosphate) and `Na₂HCO₃` (sodium bicarbonate)
- Pair II: `Na₂CO₃` (sodium carbonate) and `NaOH` (sodium hydroxide)
- Pair III: `NaOH` and `NaH₂PO₄`
- Pair IV: `NaHCO₃` and `NaOH`

2. **Analyze Pair I: `NaH₂PO₄` and `Na₂HCO₃`:**
- Sodium dihydrogen phosphate can react with sodium bicarbonate. The reaction can lead to the formation of sodium carbonate and water. Therefore, these two compounds can react with each other.

3. **Analyze Pair II: `Na₂CO₃` and `NaOH`:**
- Sodium carbonate is a basic salt and sodium hydroxide is a strong base. They do not react in a way that would prevent them from existing together in solution. Thus, this pair can coexist.

4. **Analyze Pair III: `NaOH` and `NaH₂PO₄`:**
- Sodium hydroxide (a strong base) can react with sodium dihydrogen phosphate (an acid) to form sodium phosphate and water. This means they cannot exist together in solution.

5. **Analyze Pair IV: `NaHCO₃` and `NaOH`:**
- Sodium bicarbonate (a weak acid) can react with sodium hydroxide (a strong base) to form sodium carbonate and water. Hence, this pair also cannot coexist in solution.

6. **Conclusion:**
- The pairs that cannot exist together in aqueous solution are:
- Pair III: `NaOH` and `NaH₂PO₄`
- Pair IV: `NaHCO₃` and `NaOH`

### Final Answer:
The pairs of compounds which cannot exist together in aqueous solution are (III) `NaOH and NaH₂PO₄` and (IV) `NaHCO₃ and NaOH`.