Or there is the most fingummy diplomatic note on record: when Philip of Macedon wrote to the Spartans that, if he came within their borders, he would leave not one stone of their city, they wrote back the one word"”"If"
Options (b) and (c) are ruled out because 'rude' or 'simple' statements can never be diplomatic.
'Witty' is also ruled out as witty remarks must contain an element of humor. So the correct option is 'terse' which means to the point, laconic