Which sentence shows the correct use of a comma in direct address? a) are you going to the beach this weekend walt? b) lincoln, our 16th president, was assassinated on april 14, 1865. c) denise, could you call me tonight to discuss our biology project? d) our team for the golf tournament is matthew, david, john, and alex.

Only two of the four sentences contain a direct address.  A direct address exists when someone addresses someone directly, meaning speaks to them by name.  In (A), Walt is being addressed.  In (C), Denise is being addressed.  Direct addresses are always set off (separated from the other words) by commas.  If the direct address is the first word in a sentence, it is followed by a comma.  If the direct address is the last word in a sentence, it is immediately preceded by a comma.  If the direct address appears elsewhere in the sentence, it is sandwiched between commas.  For example,  "I was wondering, David, if you'd mind helping at the event?"  By the way, when a title or endearment functions in a sentence as a name, it is also considered a direct address:  "If you don't let me go to the party, Mom, I'll never speak to you again."  In your problem, (C) contains the direct address that is punctuated properly because Denise is followed by a comma.