Hi Kapil,
The Custom Metadata Type has a suffix of __mdt instead of __c (for Custom objects/fields).
Custom metadata type field names have a suffix of __c, like other custom fields.
You can do query like this
SELECT Entity_Relationship__c, Field_Relationship__c, Default__c FROM Source_Default_Value__mdt WHERE Entity_Relationship__c = ‘Account’
For more you can refer to the standard salesforce guide “Custom Metadata Types
Implementation Guide” and practise this on Trailhead “Custom Metadata Types”.
Hope this helps you.