as usual, we will consider a list of possible pronunciations of all the options above and the one with different sound from the rest will be picked. 

rough----[rʌf]

dog----/dɒɡ/

cough---/kɒf/

rot----/rɒt/. From the outlined pronunciations, we can see that only ''rough'' has a different sound ''ʌ'' from the other options with ''ɒ''.