Answer:
a^12+8
Step-by-step explanation:
let's check all the expressions
1. a^3+18
here 18 is not a perfect cube.
2. a^6+9
here 9 is not a perfect cube.
3. a^9+16
here 16 is not a perfect cube.
4. a^16+8
here 8 is a perfect cube, thus we can write above expression by using laws of exponents as
(a^4)^3+2^3
Thus, is a sum of cubes.
a^12+8
