The correct answer is A. 70 dB.
Common-mode rejection ratio (CMRR) is a measure of an op-amp’s ability to reject common-mode signals. Common-mode signals are signals that are present on both inputs of the op-amp. CMRR is usually specified in decibels (dB). A higher CMRR indicates that the op-amp is better at rejecting common-mode signals.
A typical IC op-amp has a CMRR of 70 dB. This means that the op-amp can reject common-mode signals that are 70 dB lower than the differential-mode signals. For example, if the differential-mode signal is 1 V, the op-amp can reject common-mode signals that are as high as 70 dB below 1 V, or 0.01 V.
The other options are incorrect. Option B, 80 dB, is a higher CMRR than a typical IC op-amp. Option C, 85 dB, is an even higher CMRR than option B. Option D, 75 dB, is a lower CMRR than a typical IC op-amp.