A.) 29.3 has three significant digits because it contains no zeros on the ends.
If a number has zeros on the ends, they are just placeholders, such as 290. The 2 and the 9 are significant digits, but that 0 on the end is just a placeholder.
0.029 also has only 2 significant digits, the 2 and the 9 are significant digits, but the 0s on the ends are just placeholders.
However! If a number has a 0 on the inside, it is considered significant... so 209 has 3 significant digits, where 290 only has 2.