The correct answer to the question is parallel.
EXPLANATION:
Before answering the question, first we have to understand a longitudinal wave.
A longitudinal wave is a type of mechanical wave in which the particle vibration is parallel to the direction of wave propagation.
In this type of wave, one will find compression and rarefaction . Compression is the high pressure region where particles are closely aggregated to each other while rarefaction is the low pressure region where particles are far away from each other. Hence, a longitudinal wave is a series of compression and rarefaction.
The transfer of energy occurs from particle to particle. On the other hand, we may say that transfer of energy takes place from compression to rarefaction, and this process is repeated.
Hence, the longitudinal waves transfer energy parallel to the direction of the wave motion.
