For me at least option b and option d looks pretty much similar.
option B.
Query 1 will return all rows of table 1 and Query 2 will return only rows where condition matched Flag = 1
Option D
Query 1 return all rows of table 2 where Flag =1 with all rows of table 1 while Query 2 only return rows from table 1 and table 2 where FLAG = 1