My father has asked where my mom is. She passed away 13 years ago. How do I answer this question?

Don't answer that question over and over -- it'll just upset him, because it'll be as though he's finding out for the first time each time you do.

Diverting his attention is a better option. Try "she's not here right now," and go on to some other topic.

Sometimes asking "where do you think they are?" is a good strategy, they might surprise you by responding with the correct answer....
"Oh, she died didn't she." "I know you miss her dad, so do I. Remember when..."

But when they don't you can use their response to frame the conversation....
"I suppose she's gone to one of her meetings again" "Yes, she's always busy doing something, lets just watch your program for now"....

If the answer that she is dead is disturbing and causing him to be upset and grieve again, I would not tell him that she's gone. I would be kind and tell him some vague statement that she went to the store or is at an appointment. Whatever, just no need to upset him over and over.

Gently and truthfully and likely over and over again, as he will not remember. Their minds go into a loop with questions often enough. Eventually another subject may displace this one. I hope so.

My 95 year old Mom who has dementia / Alzheimer's will ask me where her husband (my Dad) is and I tell her the truth that he passed away. She responds "oh that's right, I forgot"... Then she'll add that he was a good man, a good husband and father.

I find the truth is always better.

Good luck,
Jenna