Generally speaking the letter "K" doesn't occur in the Gaelic language. Hence knock, meaning hill, is properly spelled "cnoc". Knap (as in Knapdale) is cnap, and kilt is cilt. Now if one was writing exclusively in Gaelic, then the "C" version of the word would be correct. However, if one is writing in English proper usage would be the "K" version-- the "C" version being a rather precious affectation. Undoubtedly there will be some on this forum who disagree for linguistic, or nationalistic reasons, or just because they are bloody minded about it. All of which is fine. But it does not change the conventions of the use of foreign words in the English language.