Black Box testing is done

Black-box testing is a software testing technique that validates a system purely from its external behaviour: given a specified set of inputs, does the system produce the expected outputs at its interfaces? The tester has no visibility into, and makes no use of, the internal source code, algorithms, or control-flow logic - only the declared inputs and the observed outputs matter.

Applying this to the given completions of "Black Box testing is done...", the phrase "to show that s/w is operational at its interfaces i.e. input and output" is exactly this input-output, interface-level validation carried out with no reference to the internal code - which is precisely the definition of black-box testing.

Contrasting the remaining completions by their own value confirms this:

• "to examine internal details of code" describes white-box (structural) testing, the complementary technique that inspects internal logic and code paths - the opposite of what black-box testing does.

• "at client side" describes a testing location/stage (such as acceptance testing performed for or by the customer), not what a testing technique inspects; location is unrelated to the internal-vs-external distinction that defines black-box testing.

• "none of above" does not hold, since the completion "to show that s/w is operational at its interfaces i.e. input and output" is a precise match to the black-box testing definition.