When you ask to write the correct number of significant fiqures for 2.35 * 10^4 are you asking for the answer 23,500
The number of significant figures in 2.35 * 10^4 is three. The 2, the 3 and the 5 are significant figures. When you write 23,500 the 2, 3 and 5 are again significant figures but the two zero's are not.
2.35 * 10^4 implies that the quantity was observed to 3 significant figures, and that only these three figures are significant.
Had the quantity been reported as 2.350 * 10^4, then the 0 would be considered significant. There is no good way to signify this with the form 23,500 so in this case only the scientific notation would tells you the number of significant figures.
2.3500 * 10^4 would indicate that both zeros are significant, and the expanded number could be written 23,500. with the . at the end indicating that everything up to that point is significant.