The correct answer is option 'A': Sinhalese and Tamils.

Explanation:
Sri Lanka is a diverse country with multiple ethnic and religious groups. The two major social groups in Sri Lanka are the Sinhalese and Tamils. Let's discuss each group in detail:

1. Sinhalese:
The Sinhalese people are the majority ethnic group in Sri Lanka, accounting for approximately 74% of the population. They are primarily concentrated in the central, western, and southern parts of the country. The Sinhalese have their own language called Sinhala and follow the Buddhist religion. Buddhism plays a significant role in their culture and traditions.

2. Tamils:
Tamils are the largest minority ethnic group in Sri Lanka, making up around 15% of the population. They are mainly concentrated in the northern and eastern regions of the country. The Tamil community is further divided into two subgroups: Sri Lankan Tamils and Indian Tamils. Sri Lankan Tamils have been living in Sri Lanka for generations, while Indian Tamils are descendants of laborers brought from South India during the colonial period. Tamils have their own language called Tamil and follow various religions, including Hinduism, Christianity, and Islam.

The Sinhalese-Tamil divide in Sri Lanka has been a longstanding issue that has led to ethnic tensions and conflicts. The root causes of these tensions include language policies, cultural differences, political representation, and territorial disputes. The most notable conflict was the Sri Lankan Civil War, which lasted for nearly three decades and ended in 2009. The war was fought between the Sri Lankan government, predominantly Sinhalese, and the Liberation Tigers of Tamil Eelam (LTTE), a separatist militant group fighting for an independent Tamil state.

It is important to note that while Sinhalese and Tamils are the two major social groups in Sri Lanka, there are also other minority ethnic groups such as Muslims, Malays, Burghers, and various indigenous communities, each with their own distinct cultures, languages, and religious practices.

The major social groups found are the Sinhala speakers and Tamil speakers. Sri Lankan Tamils are further classified into two groups -Sri Lankan Tamil and Indian Tamils.