Step 1: Recall the correct phrase.
The common expression in English is "proud of," not "proud in/from/to."
Step 2: Analyze the options.
(A) in – Incorrect, unnatural in this context.
(B) of – Correct, "proud of our culture" is standard usage.
(C) from – Incorrect, not used here.
(D) to – Incorrect, does not fit.
Step 3: Conclusion.
Hence, the correct preposition is (B) of.