The spellings of the sound [k] are more complicated than those of any other consonant sound in English: In fact, according to the analysis I prefer, the sound [k] has seventeen different spellings. But if we concentrate on choosing between just the <c> and <k> spellings, there are some useful things to remember:
The sound [k] is never spelled <c> before the letters <e>, <i>, or <y>, because before <e> or <i> or <y> the letter <c> is always pronounced “soft”, as the sound [s], as in lace, lacing, lacy. So before <e>, <i>, or <y>, always pick <k> rather than <c>. Before <a>, <o>, <u>, or another consonant usually pick <c>, as in cap, cop, cup, crop, clip.