Why would be the D? Standard Objects can't be on the detail side, either the question is wrong or the answers are wrong.

I think the correct wording is from "child" to "master"... So, being that in this case we need Engineering_Support__c to be the child and Opportunity to be the master, in order to have one Opp related to many ES's and one ES to only one Opp. In order to setup this relationship you have to navigate to the ES object and do the setup there, you always to the setup in the object that will be on the detail side (child). Also, in order to setup a rollup summary field on the Opp that aggregates data from the ESs it's a requierment that Opp is the master and ES the child, to see it available.

Option A is the correct choice because when a Master-detail field is created from the parent Opportunity object to the child Engineering_Support__c object, it enforces a one-to-many relationship where each Engineering_Support__c record can be associated with only one Opportunity record. This ensures that each Engineering_Support__c record is related to a specific Opportunity record, and multiple Engineering_Support__c records can be associated with a single Opportunity record.

if it's going to be shown in the Opportunity then the field is a roll-up summary on the master. isn't the supposed to be letter A?

I thought standard object cannot be detailed of the master details relationship