Answer:
It would have 2 significant figures
Step-by-step explanation:
Since 2.5x10^-9 is really just 2.5 with the decimal moved 9 places to the LEFT (since it is a negative 9). It would really be 0.0000000025 but since we know that zeros in front of numbers never count as sig figs and are just place holders either way you write it, it will always be 2 sig figs.