D. SELECT Id, (SELECT Id FROM Trainers__r) FROM Gym__c WHERE Name = 'Viridian City Gym'

https://developer.salesforce.com/docs/atlas.en-us.soql_sosl.meta/soql_sosl/sforce_api_calls_soql_relationships_and_custom_objects.htm#sforce_api_calls_soql_relationships_and_custom_objects
needs an __r
if the plural of Trainer was defined as Trainers then the query would be:
SELECT Id, (SELECT Id FROM Trainers__r) FROM Gym_C WHERE Name . Viridian City Gym'

Aswer us A. D missing __r.

It should have been
SELECT Id, (SELECT Id FROM Trainers__r) FROM Gym_C WHERE Name . Viridian City Gym'