Chapter 5 - Number Theory and the Real Number System - 5.1 Number Theory: Prime and Composite Numbers - Exercise Set 5.1 - Page 256: 69

$d=8$

A number is divisible by 9 if the sum of its digits is a multiple of 9. Add the digits of the number to obtain: $=1+2+3+4+d \\=10+d$ Since $0 \le d \le 9$, then the only possible value of $10+d$ that is a multiple of 9 is 18. Thus, $d$ must be 8.