Because the first box: some boxes have no cash, and the seventh box: all boxes have cash, so one and seven are contradictory;

The second box: There are diamonds in the fourth box. The fourth box: There are no diamonds in this box. So two and four are contradictory;

The third box: all the boxes are filled with gold. Fifth box: There is no gold in some boxes.

That is, one, two, three, four, five and seven boxes must have three truths and three falsehoods.

And four of these seven sentences are false, then the sixth box is false without a tick, that is, there is a tick in the sixth box.

Therefore, choose: d.