Final answer:
In assembly language, the 'add' instruction computes the sum of two values. For the code 'add x9,x0,x1', the value of register x9 is the sum of values in x0 and x1, meaning x9 = x0 + x1, which is option A.
Step-by-step explanation:
The assembly code instruction add x9,x0,x1 indicates an addition operation. According to this instruction, the value of x9 is computed as the sum of the values in registers x0 and x1. Therefore, the correct answer is: x9 = x0 + x1, which corresponds to option A) x9 = x0 + x1.