Step 1: List what several financial institutions have in common:
Post offices, NBFCs and cooperative credit societies all take deposits and lend — so these two activities cannot be the test for "bank-ness".
Step 2: Isolate the legally exclusive feature:
Under banking regulation, only banks may issue chequable, on-demand (demand) deposits — this is the feature the law reserves for banks alone, because it lets banks create derivative deposits (credit creation).
Step 3: Conclude:
Since demand deposits are the exclusive, legally-defining feature, that is the correct answer, not general deposit-taking, lending, or time deposits (all shared with non-banks).
Final Answer:
Option C.