This is a common question! However, students are not expected to know until you tell them.
There are some patterns about when the phonograms K, C, or CK are more common, and some of them are not permitted in certain contexts, but there is not a specific spelling rule to know when you must use one of these three phonograms or another.
For this reason, it is vital to cue these phonograms whenever you hear the sound /k/ in a word. For a more detailed explanation see: How should my student know whether to use EE or EA in a word (or K, C, or CK; or...)?
Did you know?
K is the most common spelling of the sound /k/ after a consonant, after a multi-letter vowel, and after a long vowel sound.
C is the most common spelling of the sound /k/ at the beginning of the word.
K is used to spell words at the beginning of a word primarily before an E, I, or Y.