Which of the following issues would most likely be a domestic policy matter?

a. declaring war

b. meeting with the prime minister of Great Britain

c. revising campaign finance laws

d. supervising democratic elections in Iraq

"Domestic" in this context refers to national matters - those which involve activity within only one nation. Options A, B, and D involve more than one country - war being armed conflict between multiple nations and the latter two involving one country and Great Britain and Iraq, respectively.

Therefore, the answer must be option C - revising campaign finance laws.