To find the current user's default record type for a given object, such as Opportunity, the developer can use the Opportunity.SObjectType.getDescribe().getRecordTypeInfos() method to get a list of RecordTypeInfo objects that represent the record types available to the current user1. Then, the developer can iterate through the list and check the isDefaultRecordTypeMapping() method on each RecordTypeInfo object to determine which one is the default record type for the current user2. The default record type is the one that is automatically assigned to new records created by the user, unless the user selects a different record type3.
The developer cannot use the other options to find the current user's default record type. Option A is incorrect because there is no such method as profile.Opportunity.getDefaultRecordType(). Option C is incorrect because there is no such class as Schema.userInfo. Option D is incorrect because it does not find the default record type before inserting the record, but rather after creating the record, which may not be the same as the default record type. References:
* RecordTypeInfo Class | Apex Reference Guide | Salesforce Developers
* Get Default Record Type Id & Name in Apex - Salesforce Stack Exchange
* Set Record Type Preferences - Salesforce Help