181. Correction: in which
This is a hard one to explain because we need to understand several different things about English grammar in order to be able to choose the right answer. In a nutshell, we need a preposition here between the with the head noun and the sentence the surface of metal is first covered with an acid and then inscribed with some sharp instrument.
In other words, the sentence that comes from the adjective clause would look like this:
The surface of metal is first covered with an acid and then inscribed with some sharp instrument in [this] process.
TestMagic has two helpful strategies related to this question. Click below to read them:
TestMagic Trick #1
TestMagic Trick #2