Dipolar (Zwitterion) Structure:
A dipolar (zwitterion) structure refers to a compound that exists in a form where it possesses both positive and negative charges within the same molecule. This occurs when a compound contains both acidic and basic functional groups, resulting in the formation of an internal salt.

Analysis of Options:
Let's analyze each given option to determine which compound can exist in a dipolar (zwitterion) structure.

Option A: CHCHCH(N=CH)COOH
This compound does not contain both acidic and basic functional groups. Therefore, it cannot exist in a dipolar (zwitterion) structure.

Option B: (CH)CH.CH(NH).COOH
This compound contains both an acidic group (COOH) and a basic group (NH). It can exist in a dipolar (zwitterion) structure. The NH group can donate a proton (H+) to the COOH group, resulting in the formation of an internal salt with both positive and negative charges.

Option C: CHCONHCHCOOH
This compound contains an amide group (CONH), which is not a basic functional group. Therefore, it cannot exist in a dipolar (zwitterion) structure.

Option D: COOH.CHCH.CO.COOH
This compound contains two carboxylic acid groups (COOH), which are acidic functional groups. However, it does not contain a basic functional group. Therefore, it cannot exist in a dipolar (zwitterion) structure.

Conclusion:
Among the given options, only compound B, (CH)CH.CH(NH).COOH, can exist in a dipolar (zwitterion) structure because it contains both an acidic group (COOH) and a basic group (NH). The NH group can donate a proton to the COOH group, resulting in the formation of an internal salt with both positive and negative charges.