The letter c is pronounced [k] :
- when followed by a, o, u or a consonant*
- at the end of a word
Examples
ca-: car, cast, recall;
co-: coat, copper, accomplish;
cu-: cut, acute, accurate;
c+consonant: article, across, respect;
-c: tunic; arithmetic, havoc.
* consonants are the letters of the alphabet without the vowels (a, e, i, o, u), so for English they are b, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, w, x, y, z.