You can multiply in any order if it's all multiplication.
So you can do (4.5*6)*(10^-8*10^-4)
which equals 27*10^-12
Written in scientific notation is 2.7 * 10^-11 (Since you moved decimal one back, you don't have to move it again in the exponent, hence it becomes 10^-11)
D. is the answer