Basically, か , its pronunciation is 'ka'. But I hear 'ga', が , in this expression which is in the end of a question. So which one should we choose here? And when do we choose 'ka', when 'ga'? Thank u !
When reading a written sentence, the reading is as written: か is read ka, が is read ga.
As for choosing which one to use:
か is a flat out interrogative particle, marks the sentence as a regular question.
When there is ですが, this is the sentence connecting particle が with the meaning of "but" (sometimes also has introductory meaning) ... in this case, if nothing is said afterwards, there is omitted sentence that the listener should be able to guess based on context.
Common usage is to soften a request or a statement. Examples (that would work in some contexts):
~ I would have a question, but ... (don't want to bother you, would it still be alright to ask?) = you would like to ask something, but want to be polite and not push the listener into answering, so you just suggest that you have a question so the listener can react on their own volition if they want to answer (at least this is how the situation is presented)
~ I think we should do it, but ... (maybe you think otherwise, I may be wrong).
The parts in brackets may or may not have been said out loud.
Let us know if you have a specific sentence where you are unsure