question mark, ?
乎 centers on marking a question or exclamation: it functions as a classical Chinese question particle, similar to a question mark.
乎 is an ancient pictograph representing breath or a call, later used as a grammatical particle for questions and exclamations. The exact historical development is uncertain.
The shape 乎 looks like a stylized question mark with a hook at the bottom, as if the stroke is curling into a question. Think of it as a written question mark.
For コ, imagine a person asking a question with a curious 'co?' sound: co -> コ, like the start of 'question' but with a 'k' sound.