The solution that an app builder should use for this request is a lookup field. A lookup field is a type of relationship field that links two objects together and allows users to select a record from another object. The app builder can create a lookup field on the account object that references the user object and allows users to assign a customer service manager (CSM) to the account. Option A is incorrect because a multi-select picklist field is not suitable for this request, as multi-select picklist fields allow users to select multiple values from a predefined list, not from another object. Option B is incorrect because a picklist field is not suitable for this request, as picklist fields allow users to select one value from a predefined list, not from another object. Option D is incorrect because a text field is not suitable for this request, as text fields allow users to enter any alphanumeric characters, not from another object.