Some Basic Concepts of Chemistry

Let's find the number of significant figures carefully.
(A) 0.0025
Leading zeros are not significant.
Only the digits 2 and 5 are significant.
Significant figures =2
(B) 500.0
Trailing zeros after the decimal point are significant.
500.0⟶ digits 5, 0, 0, and 0 after decimal.
Significant figures =4
(C) 2.0034
All nonzero digits are significant.
Zeros between nonzero digits are significant.
So digits: 2,0,0,3,4⟶ all count.
Significant figures = 5
Final Answer
(A,B,C)⇒(2,4,5)