Let Parts Table( i am ignoring the attributes which are not neede)
pid:integer color:string
1 blue
2 green
SELECT P.pid FROM Parts P WHERE P.color<>’blue’ > gives pid2
Let Catalog (sid:integer, pid:integer)
10 1
11 2
13 1
13 2
SELECT C.sid FROM Catalog C WHERE C.pid NOT IN (pid2)gives 10, 13
Let suppliers table sid(10, 11, 12, 13)
SELECT S.sname
FROM Suppliers S
WHERE S.sid NOT IN (10, 13) gives 11, 12
11 is the supplier who supplied green, 12 is the supplier not supplied anything
10 is the supplier supplied blue part, 13 is the supplier supplied blue and green part.
we got 11, 12

Find the names of all suppliers who have supplied a nonblue part. ( a implying 11,13 wrong not matching with 11,12)

Find the names of all suppliers who have not supplied a nonblue part.  ( b implying 10, 12)

Find the names of all suppliers who have supplied only nonblue part. wrong eventhough 12 not supplied anything we got it( c implying we should get only 11 but we got 11, 12)

Find the names of all suppliers who have not supplied only blue parts.( d implying 11 and 12 not supplied blue parts so it is matching with our query)
So i think d is the answer