A tenth is a 1/10 of something, in this case, 1/10 of a gram. A tenth means that a whole is divided into ten equal parts.
Therefore, the right answer to how many tenths make a gram is:
10. Ten tenths make a gram
To give you another example, if you want to know how much is a tenth of a mile then you have to divide a mile by ten. A mile is 1.60934 kilometres, therefore a tenth (1/10) of a mile is 0.160934 kilometres. If we measure it in feet, it would be 528 feet.