Answer: B.The explanation that she had given them seemed too horrible to be true.
Step-by-step explanation:
A restrictive clause simply modifies the noun before it. It gives more information about the noun. It can either identify or limit such nouns.
The sentence with a restrictive clause include:
A. Mr. offer, whose family lives in Germany, will be our guest this weekend.
C. My father's friend who was born on the frontier became President.
In the above cases, the restrictive clause in A "whose family lives in Germany" and in B "who was born in the frontier" gives more information about the noun and identifies them.
Therefore, the answer is option B as it doesn't have a restrictive clause.