If $A\%B=A+B-1$, then what is the value of $10\%2$?

The given mathematical operation is defined as:
$$A\%B=A+B-1$$

To find the value of $10\%2$, substitute $A=10$ and $B=2$ into the definition:

$$10\%2=10+2-1$$

$$10\%2=12-1=11$$

Therefore, the correct option is A.