Answer
wave $3$
Work Step by Step
When the four unit peak in wave 1 is added to the one unit trough in wave 3, a three unit peak will result when the waves are summed (4-1=3). When the four unit peak in wave 3 is added to the one unit trough in wave 1, a three unit peak will result when the waves are summed (4-1=3). Therefore, a three unit peak that is $2d$ wide will result when waves $1$ and $3$ are summed at a certain point in time.
