Basically this is a weaken the argument question.

IF we do know from the information in the passage that X occurs, then if X will NOT occours we will weaken the argument

As rock interfaces are crossed,

The roks are crossed by signals

the elastic characteristics encountered generally change abruptly,

based on the rock elasticity the waves changed direction. I.E the rock is really solid the waves changes is > when a rock is more elastic because the impact is absorbed more

which causes part of the energy to be reflected back to the surface,

hard rock= wave more reflection back. I.E a greater bounce

elastic rock= smaller bounce back


where it is recorded by seismic instruments.

we record all these data


The seismic records must be processed to correct for positional differences between the source and the receiver, for unrelated wave trains,

the hypothetical line between the sender (rock) and the receiver (the radar) is balanced t avoid devisations


(C) If the rock on one side of a rock interface had similar elastic characteristics to those of the rock on the other side

If the different rocks are similar, our waves are similar and our data to process are similar. If all these data are alike in meaning we are not able to spot the differences.

therefore, we will NOT be able to see the differences among rocks and we are not able to find what we are looking for

I hope now is clear.
You must read carefully. Thje answer is there. Pretty straight