"か" is the question mark in Japanese writing system because we do not use "?" in formal documents. If you have a textbook, please have a look at the Japanese writing. Sentences should finished with "。" even if they are interrogative sentences. Thus, technically speaking ”か” and "?" do not exist at the same time, as the sentences with "か" are formal writing and without "か" are informal ones.

か turns it into a question. e.g. いいです ii desu it is good いいですか ii desu ka is it good? / is it okay? When it ends with か we know it's a question, a question mark is optional. :)