Step 1: Understanding the Concept:
An ethno-national identity is a sense of belonging to a group that shares an ethnic base and wants a political voice, such as a state or autonomy. In India these identities grew around shared language, a shared region and tribal community.
Step 2: Key Approach:
We test each of the four bases and see which ones the textbook treats as sources of such identities.
Step 3: Check A, Language:
Language gave rise to strong movements, for example the demand for states on linguistic lines. So A is included.
Step 4: Check B, Religion:
Religion is usually discussed in the context of communalism, and not as an ethno-national base in this unit of study. So B is left out here.
Step 5: Check C, Region:
Region shapes identity strongly, as in regional movements within the Indian Union. So C is included.
Step 6: Check D, Tribe:
Tribal communities, for example in the north-east and in central India, have raised demands built on tribal identity. So D is included.
Step 7: Choose the option:
A, C and D are the bases that fit, which is option 3. This answer is based on the usual treatment in the course, and options 1, 2 and 4 each carry religion.
Final Answer:
Language, region and tribe are the sources, which gives option 3.
\[ \boxed{3} \]