How many four-digit natural numbers are there such that all of the digits are even?

Even digits are 0, 2, 4, 6, 8 (5 choices). For a 4-digit number, the first digit cannot be 0, so 4 choices (2,4,6,8). Each of the remaining 3 digits has 5 choices. Total = 4 × 5 × 5 × 5 = 500. Wait, that gives 500. Let me recheck: 4 × 5³ = 4 × 125 = 500. Answer is (b) 500.