A good way to handle a question like this is to separate what the passage actually claims from what each option is claiming, since the two often differ only in degree.
Since only the extent claim in option C lines up with what the passage actually says, that is the correct choice.
There is a useful piece of background here: British officials in the colonial era sometimes dismissed the Indian National Congress as representing only a microscopic minority of India, a phrase used to deny that Congress spoke for ordinary Indians. This passage reads like a direct answer to that old charge, so checking the options against that context sharpens the choice.
Read against that historical backdrop, the passage is making the case against option D and for option C, so the correct answer is option C.
Instead of relying on the word dumb alone, look at the sentence it sits in: the capacity of the poor dumb millions of India to take part in modern nationalist politics was also demonstrated. The sentence is about people who had not previously been part of political life being drawn in for the first time.
That reading matches option D, so option D is correct.
This question has a built in trap: reading a modern meaning into a phrase from older nationalist writing. Checking each option for that trap makes the right choice clearer.
Once the older, figurative sense of dumb is used instead of the modern one, option D is clearly the correct answer.
One clue that is easy to miss is the regretful note near the end of the passage, where it says this positive feature of the movement was unfortunately not repeated in later years once communalism took its toll.
The regretful tone about a good thing not lasting only makes sense if that good thing, communal unity, truly happened, which points to option C as correct.
The passage adds a specific detail worth unpacking: it says communal unity was maintained despite the Malabar developments, a reference to the communal tensions that broke out in Malabar, present day Kerala, during this period. Reading that detail helps settle the options.
Once the Malabar reference is understood as a near test of unity that the movement withstood, option C stands out as the true statement.
A quick way to sort main idea options is to note what is wrong with each one that fails, since main idea questions usually fail for one of a few standard reasons.
With the contradicted, unsupported, and reversed options removed, option B remains as the accurate main idea.
The main idea of a passage has to hold across all of its parts, not just one paragraph, so testing each option against every paragraph is a reliable way to find it.
Only option B is consistent with every paragraph of the passage, which makes it the correct summary of the main idea.
The passage uses communal unity and communalism close together, and reading them as a pair makes the meaning clear.
Read this way, the word points to religious identity, confirming option A.
English has a trap word here: communal in everyday use can mean simply shared or collective, like a communal kitchen, but communalism in Indian political history is a specific term with its own meaning, and the options should be tested against the historical term, not the everyday one.
Once the everyday sense of communal is set aside for the specific historical term, option A is the only option that matches, making it the correct answer.