The correct answer is B. of.
The sentence is “We have run out of sugar.” This means that we no longer have any sugar. We can use the preposition “of” to indicate that we have used up something. For example, we can say “I ran out of gas” or “We ran out of time.”
The other options are not correct because they do not make sense in the context of the sentence. The preposition “for” is used to indicate a purpose or destination. For example, we can say “I went to the store for milk.” The preposition “in” is used to indicate a location or position. For example, we can say “The sugar is in the cupboard.” The preposition “to” is used to indicate a direction or destination. For example, we can say “I am going to the store.”