The pair `NH_(3)` and `BH_(3)` is isoelectronic with

Options

A. `B_(2)H_(4)`
B. `C_(2)H_(6)`
C. `C_(2)H_(4)`
D. `CO_(2)`

Explanation

To determine which species is isoelectronic with the pair NH₃ and BH₃, we need to calculate the total number of electrons in each of these molecules and then find another species that has the same total number of electrons.

### Step 1: Calculate the number of electrons in NH₃
- **Atomic number of Nitrogen (N)**: 7 (which means it has 7 electrons)
- **Atomic number of Hydrogen (H)**: 1 (each hydrogen has 1 electron)
- **Total number of electrons in NH₃**:
\[ \text{Electrons in NH₃} = 7 \text{ (from N)} + 3 \times 1 \text{ (from 3 H)} = 7 + 3 = 10 \]

### Step 2: Calculate the number of electrons in BH₃
- **Atomic number of Boron (B)**: 5 (which means it has 5 electrons)
- **Total number of electrons in BH₃**:
\[ \text{Electrons in BH₃} = 5 \text{ (from B)} + 3 \times 1 \text{ (from 3 H)} = 5 + 3 = 8 \]

### Step 3: Calculate the total number of electrons in the pair NH₃ and BH₃
- **Total electrons in NH₃ and BH₃**:
\[ \text{Total electrons} = 10 \text{ (from NH₃)} + 8 \text{ (from BH₃)} = 18 \]

### Step 4: Find a species that has 18 electrons
Now we need to check the options provided to find a species that has a total of 18 electrons.

1. **Option 1: B₂H₄**
- Electrons: \(2 \times 5 \text{ (from 2 B)} + 4 \times 1 \text{ (from 4 H)} = 10 + 4 = 14\) (not isoelectronic)

2. **Option 2: C₂H₆**
- Electrons: \(2 \times 6 \text{ (from 2 C)} + 6 \times 1 \text{ (from 6 H)} = 12 + 6 = 18\) (isoelectronic)

3. **Option 3: C₂H₄**
- Electrons: \(2 \times 6 \text{ (from 2 C)} + 4 \times 1 \text{ (from 4 H)} = 12 + 4 = 16\) (not isoelectronic)

4. **Option 4: CO₂**
- Electrons: \(1 \times 6 \text{ (from C)} + 2 \times 8 \text{ (from 2 O)} = 6 + 16 = 22\) (not isoelectronic)

### Conclusion
The species that is isoelectronic with NH₃ and BH₃ is **C₂H₆**, as it has the same total number of electrons (18).

### Final Answer
**C₂H₆**