(b) is the correct answer i.e. b1
According to the given diagram if we draw a table then it should be something like below
Table for Entity A:-
a1 (attribute and primary key) |
a2(attribute) |
a11 |
a21 |
a12 |
a21 |
a13 |
a23 |
a14 |
a24 |
Table for Entity B:-
b1(attribute and primary key) |
b2(attribute) |
b11 |
b21 |
b12 |
b22 |
b13 |
b21 |
b14 |
b22 |
b15 |
b21 |
Table for Relation R:-
a1(attribute) |
b1(attribute) |
a11 |
b11 |
a12 |
b12 |
a13 |
b13 |
a14 |
b14 |
Note that b15 is not mapped to any one because we are considering the case of partial participation at Entity B-side.
Now According to 1:1 and Total Participation at one side, only 1 table is required let's name this table as ARB then the content of this table will be according to 1:1 and total participation at one side are as follow:-
Table for ARB:-
a1(attribute) |
a2(attribute) |
b1(attribute and primary key) |
b2(attribute) |
a11 |
a21 |
b11 |
b21 |
a12 |
a21 |
b12 |
b22 |
a13 |
a23 |
b13 |
b21 |
a14 |
a24 |
b14 |
b22 |
NULL |
NULL |
b15 |
b21 |
Now in the above table, you can see that attribute 'a1' is just unique and not a primary key because primary key can never be NULL and attribute 'b1' is unique and not NULL that's why it is the primary key of the resultant table.
Hence, the answer is "b1" as the key for the final table.