か (ka) — Question, Or, and Uncertain Identity

それは本当ですか？

Sore wa hontou desu ka?

Is that true?

Statement + か turns it into a yes/no question

ペンか鉛筆を貸してください。

Pen ka enpitsu o kashite kudasai.

Please lend me a pen or a pencil.

か between nouns means 'or'

誰か来ましたか？

Dareka kimashita ka?

Did someone come?

誰 (who) + か = 誰か (someone); the final か makes it a question. Not to be confused with 誰が (who specifically).