Answer:
They all have to do with counting.
円 EN, maru(i), yen - yen, round : Counter for mney
問 MON, to(u) - question, problem : Counter for questions
具 GU - tool : Counter for tools
客 KYAKU, KAKU - guest, customer : Counter for place settings
献 KEN, KON - present, offer : Counter for the number of times you pour sake.