It is not clear to me When should I use jemu and when should I use mu?
I am quite sure there's some rule for their usage but as a Czech person, I kinda just judge by my feeling (while knowing that's definitely not the best advice to give on a website as such :) When "jemu" is not the DIRECT, IMPORTANT part of the sentence, you would generally change the word order and just use "mu" (as in example "Řekni mu to" = Tell him). On the other hand, when it's the part you want to put stress on, you use the full form "jemu" (Řekni to jemu = Tell him (but noone else, just him)). For example in the given sentence, it's the interest of the asker to find out whether it was told to "Guy 1" or "Girl 2", so "jemu nebo jí?". If you just generally pointed at a person and wanted to ask if they have been told, you would ask "Řekl jsi mu to?").
i think it is a matter of interpretation as seizeq has explained so both answers can be correct
They can't. The word order dictates "jemu" here. With "mu" it would have to be "Řekl jsi mu to?". The question is what is the unknown new thing in the question. Are you asking whether he told him (you know whom) it or not? => "Řekl jsi mu to?" Are you asking whom did he tell that? => "Řekl jsi to jemu nebo jí?"
Did you tell him or her is marked wrong. Did report it but i was wondering when řekli means say or when it means tell.
The actual sentence i used was did i tell him or her that, but really it is a question about the use of řekli, when is it say and when is it tell. Sorry my eyesight is deteriorating and I am makings lots of silly errors