Ming placed a zero in the number 590 because the digit 1 in 16 has a value of 10. When multiplying 59 by 16, she first multiplied 59 by 6 (the units digit of 16) to get 354, and then multiplied 59 by 10 (the tens place digit of 16), which is 590. This is the reason for the zero, as it indicates that she is multiplying by 10.
So, the correct answer is: • The digit 1 in 16 has a value of 10.