The innermost query select those projects which have employees working under it; hence NOT EXISITS will select those Pid's that do not have any employees working under it. The second innermost query will select those Pid's which doesn't have any employee working under it and goes by the name 'Database'. Say 3 and 4 are such Pid's. Now, Pid 2 is also named as database. Hence the outermost query will select 2 also along with other pid's. Doesn't that make option c correct?