In a speech community, it is observed that ‘s’ is pronounced differently by two different socio-economic groups of speakers. This difference is an example of :

The correct option is A socio-linguistic variable.

Step 1: Understand the given context
In sociolinguistics, a linguistic feature (such as the pronunciation of a sound like 's') often varies depending on social factors, such as social class, economic group, age, gender, or region. When a linguistic item has different realizations (allophones or variants) correlates with social status or group identity, it acts as a marker or indicator of social identity within a speech community.

Step 2: Define the terms
- Socio-linguistic variable: A linguistic feature (like the phonetic pronunciation of 's') that correlates systematically with social variables (such as socio-economic status, class, or background).
- Socio-cultural variable: Factors related to social structure and culture broadly (e.g., traditions, social norms), not specifically tied to language variation.
- Socio-psychological variable: Factors combining individual psychology and social interaction, such as attitudes, perception, and group identity.
- Pragmatic variable: Elements of language choice based on context, intent, and conversational setting rather than social group identity.

Step 3: Conclusion
Since the variation in the pronunciation of the sound 's' directly correlates with different socio-economic groups, it represents a classic example of a socio-linguistic variable.