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 Make out : a. To understand or see something unclear  

        I can't make out the words on the sign from here. 
                      b. To engage in passionate kissing. 

They were caught making out in the park. 

Make up  :a. To invent or create a story or excuse             

              He made up a silly excuse for being late. 

                  b. To reconcile or restore a friendly relationship 

             After their argument, they decided to make up and be friends again. 

Mix up :To confuse or mistake something or someone. 

        She always mixes up our names.