Question

My mother ____ the books in the library when the guest arrived​

Answer 1

My mother has placed / has kept the books in the library when the guest arrived​ .

Explanation:

The past simple or past perfect can be used in combination with present perfect.

This seems an example of such combination the sentence can be made like this.

Incomplete sentence:

My mother ____ the books in the library when the guest arrived​ .

Complete sentence:

My mother has placed / has kept the books in the library when the guest arrived​ .