For optimizing queries, especially on large data volumes, it's recommended to use indexed fields such as External ID fields, which will make the queries selective. Among the options given:
B) is optimized because it is using the Customer_Number__c field, which is an External ID and hence indexed.
D) is also optimized because it filters out deleted records with IsDeleted = false and uses Name != ' ', which is likely to be selective.
Using indexed fields and filtering out unnecessary records helps in faster query execution on large data volumes.
References:
Make SOQL Query Selective
Working with Very Large SOQL Queries