Compression of a nerve within the carpal tunnel products inability to -
This question is tricky one.
As you all know, both abductor pollicis brevis (producing abduction of thumb) and opponents pollicis (producing opposition of thumb) are paralysed in low median nerve palsy (in carpal tunnel).
So, the opposition of thumb is lost.
However, the fact is different for the abduction of thumb (Read below).
The abduction of thumb is of two types due to movement in two different planes : -

i) Radial abduction: - Thumb is moved in the same plane as the palm. To test the radial abduction, the hand is placed on a surfade and the patient is asked to move thumb outward against the resistance. Radial abduction is executed by abductor pollicis longus and extensor pollicis brevis, the muscle supplied by radial nerve.
ii)Palmar abduction: - The thumb is moved at right angle to the plane of palm. To test palmar abduction 'pent test' is used (Pen test has been explained). Palmar abduction is executed by abductor pollicis brevis.

So, in median nerve palsy, palmar abduction will be lost, not the radial abduction.
Abduct the thumb
Adduct the thumb
Flex the distal phalanx of the thumb
Oppose the thumb