Concept:The blank requires the correct preposition in the idiomatic phrase “keep peace with,” which means to remain on peaceful or friendly terms with someone.
Explanation:The sentence says the little boy found it difficult to stay calm and friendly with his father.
The fixed expression for this situation is “keep peace with.”
Among the given words, only “with” fits this idiomatic usage.
“Against,” “by,” and “over” do not form the natural expression needed here.
Thus, the completed sentence is: “The little boy found it difficult to keep peace with his father.”
Answer:with