The requirements necessitate a one-to-many relationship where one Opportunity can have multiple EngineeringSupportc records. This means the relationship field must be created on the EngineeringSupportc object (the "many" side) and point to the Opportunity object (the "one" side). The second requirement, to show aggregate information (e.g., count, sum) from the child EngineeringSupportc records on the parent Opportunity record, is a direct feature of Master-Detail relationships via Roll-Up Summary Fields. A Lookup relationship does not natively support this aggregation capability. Therefore, a Master-Detail field from EngineeringSupportc to Opportunity is the correct implementation.

1. Salesforce Help: Object Relationships Overview. This document explicitly states

"Use a master-detail relationship to... create roll-up summary fields on the master record that aggregate the detail records." This directly addresses the aggregation requirement.

2. Salesforce Help: Create a Master-Detail Relationship. This guide clarifies the implementation: "A relationship field (which is a custom field) is created on the detail object." In this scenario

EngineeringSupportc is the detail object.

3. Salesforce Developer Guide: Relationships Among Objects. This guide differentiates the relationship types

noting that a key feature of Master-Detail relationships is that they "allow you to define roll-up summary fields." It contrasts this with Lookup relationships

which lack this capability.