The dipole moment is the highest for? 

Acetophenone 

The correct answer is option 3. Acetophenone.

To determine which of the given compounds has the highest dipole moment, we need to analyze the molecular structure and the distribution of polar bonds in each molecule.

1. trans-2-butene:

In trans-2-butene, the two methyl groups are on opposite sides of the double bond, which cancels out their dipole moments, resulting in a very low overall dipole moment.

2. 1,3-Dimethylbenzene (m-xylene):

1,3-Dimethylbenzene has methyl groups attached to the benzene ring at the 1 and 3 positions. The symmetry and the positions of the methyl groups lead to a low overall dipole moment.

3. Acetophenone:

Acetophenone \((C_6H_5COCH_3)\) has a carbonyl group \((C=O)\) attached to a benzene ring. The carbonyl group is highly polar, creating a significant dipole moment. Additionally, the phenyl ring does not significantly cancel out the dipole moment of the carbonyl group, resulting in a relatively high dipole moment.

4. Ethanol:

Ethanol \((CH_3CH_2OH)\) has a polar hydroxyl group \((–OH)\), which gives it a dipole moment. However, the dipole moment is not as high as in acetophenone because the dipole created by the hydroxyl group is not as strong as that created by the carbonyl group in acetophenone.

Given this analysis, acetophenone has the highest dipole moment among the given compounds due to the presence of the highly polar carbonyl group.

Therefore, the correct answer is Acetophenone