A Go to a health club every four days (that is, the fifth day), the first time is the first Monday in January;

B Go to the health club every three days (that is, the fourth day), the first time is the first Tuesday in January;

We don't know when the first Monday in January appears, so let's assume the most extreme situation, assuming that 65438+ 10/is Tuesday, then the first Monday in January is 65438+1 0, and the first Tuesday is 65438+ 10.

So:

A The second visit (once every 4 days) should be 65438+ 10/2 (7+4+1=12);

The second time (every 3 days) should also be 65438+ 10 month12 (8+3+1=12).

So the last meeting between Party A and Party B should be 12. So rule out BCD immediately.

Choose a.

That is, 1.2 is the first Monday in January.