Discussion
Assertion (A) : An operational amplifier should have a low input offset current.
Reason (R) : Input impedance of operational amplifier is very high.
Answer: A
The key's answer is that both statements are true and (R) is the correct explanation of (A). I could not confirm this reading in a standard source, so it follows the key.
Input offset current is the difference between the two input bias currents of an op-amp. The bias currents are small when the input stage has a very high input impedance (nanoamperes for bipolar inputs, picoamperes for FET inputs), and their difference is smaller still, which is the link the key draws between (R) and (A). A low offset current is wanted because the difference current flowing through the source resistances makes an unwanted offset voltage at the output.
(A) is therefore true, and (R) is true too. Option B would say (R) is true but unrelated; the key judges it related. Options C and D are ruled out because both statements are true.
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