I am relatively new to deciphering text and I've been given these few puzzles to solve. I tried all available deciphering tools from Black Chamber and Rumkin tools but I can't seem to solve the puzzle. So far, I managed to solve 1 puzzle which uses the letter substitution. It will be helpful if you can solve the puzzle and explain the steps so I'll understand more. Thanks in advance.
a) JH Q PHUW GQXH Q VPQMSH
c) HX VT YD HQ FV KX MI RK AU PC DR MB AB OE OP TE OD KP AD LM BW ZL AP MT NA ER UC DM CM NR SM DB QO NS