A Master-Detail relationship is the most appropriate choice. This relationship type creates a tight coupling between the parent (Opportunity) and the child (Installation) objects. The ownership and sharing of the child Installation records are determined by the parent Opportunity. This directly fulfills the requirement that sales reps have visibility into the installations associated with their opportunities without needing additional sharing configurations. The lifecycle of the Installation record is also tied to the Opportunity, which aligns with the business process where an installation only exists because of a sale.

A. Many to Many: This is incorrect as it implies an installation could be related to multiple opportunities. The scenario describes a one-to-many relationship (one opportunity, many installations).

C. Hierarchical: This is incorrect because this special relationship type is exclusively for the User object to create hierarchies, such as a reporting structure.

D. Lookup: This is less suitable because it creates a loosely coupled relationship. The Installation object would have its own sharing settings, requiring extra configuration to grant visibility to sales reps.

1. Salesforce Help, "Object Relationships Overview": This document outlines the different types of object relationships. It states, "In a master-detail relationship, the detail record inherits the sharing and security settings of its master record." This directly supports the choice of Master-Detail to control visibility based on the parent Opportunity.

2. Salesforce Help, "Create a Master-Detail Relationship": This guide details the behavior of master-detail relationships. Under the "Result of Creating a Master-Detail Relationship" section, it confirms, "The owner of the master record implicitly owns the detail and subdetail records... The sharing setting for the detail and subdetail records is determined by the master record’s sharing setting."

3. Salesforce Developer Documentation, "Relationships Among Objects": This resource provides a technical overview. It describes the master-detail relationship as one where "the master object controls certain behaviors of the detail object, such as who can view the detail’s data." This aligns perfectly with the scenario's visibility requirement.