The correct answer is 18. Powers of 10 are represented by a 1 followed by the number of zeroes that matches the power (so 10^4 would be 10,000, or 1 followed by 4 zeroes). Since 987,654,321,098,765,432 is 18 digits long, we need 10^18 to display a 1 followed by 18 zeroes, which would be 19 digits, which would exceed the 18-digit number.
