Peter and Jupiter play the guitar together on the LCM of their individual intervals i.e. LČM (3, 4) = 12.

Hence, they play the guitar together on every 12″ day.

Observe that the dates given in the options are of January and February.

The dates in these two months, when they play the guitar together are 15th January, 27th January, 8th February and 20th February.

Hence, 7th February is an invalid date.

Hence, option c.