The correct option is
A -
Answer : A
Explanation:
The Relative Pronoun can be dropped when it represents the object of the sentence but not when it represents the subject of the sentence.
Here in the given sentence:
"Bill" -- is the subject of the sentence.
"gave" -- is the verb
"dictionary" -- is the object of the sentence
"me" -- indirect object of the sentence
Here, in this question, the relative pronoun with which we are supposed
to fill the blank should correctly represent the object ("dictionary"), which is the antecedent.
But as it would be representing the object of the sentence, it can be dropped.
Moreover,
"who" is used to refer to person,
"when" is used to refer to time, *
"where" is used to refer to a place,
thus all these are inappropriate for use here anyway, and thus are eliminated.
Therefore the complete correct sentence is:
That's the dictionary Bill gave me for my birthday.