Step 1: The distance of a point $(a,b)$ from the x-axis is $|b|$, i.e. the ordinate.
Step 2: The distance of a point $(a,b)$ from the y-axis is $|a|$, i.e. the abscissa.
Step 3: Since $b>a>0$, both $a$ and $b$ are positive, so $|b|=b$ and $|a|=a$.
Step 4: Required value
$$=\text{(distance from x-axis)}-\text{(distance from y-axis)}=b-a$$
Correct option is (B) $b-a$.