Step 1: Understanding the question.
We need to choose the correct preposition to complete the sentence in a grammatically appropriate way. The phrase "express solidarity" commonly takes the preposition "with" to indicate the person or group one is supporting.
Step 2: Analyzing the options.
(A) for: This is incorrect, as "solidarity for" is not a common construction.
(B) on: This is incorrect, as "solidarity on" does not convey the correct meaning.
(C) with: This is correct, as we typically express solidarity "with" someone or a group.
(D) in: This is incorrect, as "solidarity in" does not fit in this context.
Step 3: Conclusion.
The correct answer is (C) with, as it correctly completes the sentence.