This would be known to be the (tenth) thousands.
(5)7,,634= The 5 would equal the ten thousands because first, once the number would have it's (first) comma, this would be in the thousands. And also, this number has (2) numbers in it's thousands, and not just 1. But if it were to have 3, this would be another story, and from this, it would clearly explain to be your (option (b)).
