答案：B

解析：

Choice B is correct. First, this is a linear equation ine one variable.
When a linear equation has one solution, the equation reduces to an equation of the form
ax+b=cx+d, where a does not equal c.
Rewrite the equation in an equivalent form using the distributive property and combining like terms
4(x-2)+x=ax-8
5x-8=ax-8
If a=5, the two sides of the equation are the same. In this case, there are infinitely many solutions.
If a is any number other than 5, the equation will have exactly one solution, x=0. In this case, the only correct answer is a=3.
If a=3, the equation will have exactly one solution.