Using by which

Could you please give me an explanation about using “by which” considering the question below?

The Question is "Mechanization is the process … work that was formerly done by hand is accomplished by machine.

A)which, B) that, C) that which, D) by which

According to me the answer might be “in which” but it isn’t among the preferences.

‘By which’ is correct.
It is one of a range of prepositions used with ‘which’ to form prepositional structures.
Its meaning is the same as ‘in which’ but 'in which ’ usually relates to a place, rather than a mechanism.

Although use of ‘by which’ is not specifically mentioned, you may find this information useful.

Thank you for the information.