Question

Nick's mother invented a plastic instrument. It helps Nick in holding things (combine with which)

Answer 1

Nick's mother invented a plastic instrument which helps Nick in holding things

Answer 2

It is relative pronoun case. Which is a relative pronoun used in defining or non-defining relative clause. ‘Which’ as a pronoun can only be used to refer only to things.

Defining relative clause functions  or identifies  which person or thing you are talking about. Non-defining relative clause only describes and adds extra information about somebody or something.  If left out , the sentence would still make sense. A non-defining relative clause is usually put in commas.

The correct combining of the given sentences is as given below:

Nick’s mother invented a plastic instrument which helps Nick in holding things.