Answer: Option B
Prepositions are words that show the relationship between a noun or pronoun and other words in a sentence. In the given sentence, the preposition should correctly convey the idea of having discussions on a particular matter with someone. Let's look at the given options and why they may or may not be correct:
A. About; in: This preposition doesn't fit in the context of the sentence as "about" indicates discussing the topic itself, while "in" would imply having discussions inside a particular location.
B. With; on: This preposition correctly conveys the idea of discussing something with someone and specifically discussing the matter at hand. "With" shows who the discussions are taking place with, while "on" indicates what the discussion is focused on.
C. With; for: This preposition is not suitable for the context of the sentence as "for" implies discussing the reason behind something or the intended outcome, which doesn't fit the context of having discussions on a particular matter.
D. About; for: This preposition is not the best fit for the context of the sentence as "about" indicates discussing the topic itself, while "for" implies discussing the intended outcome or benefit, which doesn't fit the context of having discussions on a particular matter.
E. None of these: This option is incorrect as option B is the most appropriate preposition to use in the given sentence.
In conclusion, option B, "with; on", is the most appropriate preposition to use in the given sentence. "With" shows who the discussions are taking place with, while "on" indicates what the discussion is focused on.