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(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.

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