To determine which compound has all carbon atoms that are `sp²` hybridized, we will analyze each option provided in the question.
### Step 1: Analyze Option 1 - CH₂=CH-CH₂
- The structure can be represented as:
```
H H
\ /
C
||
C
/ \
H C
/ \
H H
```
- In this compound, the first carbon (C1) is bonded to two hydrogens and one carbon with a double bond, making it `sp²` hybridized.
- The second carbon (C2) is bonded to one carbon with a double bond and another carbon with a single bond, making it `sp²` hybridized.
- The third carbon (C3) is bonded to two hydrogens and one carbon with a double bond, making it `sp²` hybridized.
- However, the middle carbon (C2) is involved in two pi bonds (due to the double bonds), which means it is `sp` hybridized.
- **Conclusion**: Not all carbon atoms are `sp²` hybridized in this compound.
### Step 2: Analyze Option 2 - 1,3-Butadiene
- The structure can be represented as:
```
H H
\ /
C
||
C
/ \
H C
||
C
/ \
H H
```
- In 1,3-butadiene, the first carbon (C1) is bonded to two hydrogens and one carbon with a double bond, making it `sp²` hybridized.
- The second carbon (C2) is bonded to one carbon with a double bond and another carbon with a single bond, making it `sp²` hybridized.
- The third carbon (C3) is also bonded to one carbon with a double bond and another carbon with a single bond, making it `sp²` hybridized.
- The fourth carbon (C4) is bonded to two hydrogens and one carbon with a double bond, making it `sp²` hybridized.
- **Conclusion**: All carbon atoms in 1,3-butadiene are `sp²` hybridized.
### Step 3: Analyze Option 3 - CH≡C-CH≡CH
- The structure can be represented as:
```
H H
\ /
C
||
C
/ \
H C
/ \
H C
```
- The first carbon (C1) is involved in a triple bond, making it `sp` hybridized.
- The second carbon (C2) is also involved in a triple bond, making it `sp` hybridized.
- The third carbon (C3) is involved in a triple bond, making it `sp` hybridized.
- The fourth carbon (C4) is also involved in a triple bond, making it `sp` hybridized.
- **Conclusion**: Not all carbon atoms are `sp²` hybridized in this compound.
### Step 4: Analyze Option 4 - Cyclohexane
- The structure can be represented as:
```
H H
/ \
C C
/ \
H H
| |
C C
\ /
H H
```
- In cyclohexane, all carbon atoms are bonded to two hydrogens and two other carbons with single bonds, making them `sp³` hybridized.
- **Conclusion**: Not all carbon atoms are `sp²` hybridized in this compound.
### Final Conclusion
The only compound where all carbon atoms are `sp²` hybridized is **1,3-butadiene**.
### Answer: Option B - 1,3-butadiene
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