To find the highest power of x in the given expression, we need to determine the highest power of x that appears in each factor and then take the maximum of those powers.
In the expression (x-a)(x-b)(x-c)....(x-z), each factor (x-a), (x-b), (x-c), ..., (x-z) is a linear term. The highest power of x in each of these factors is 1.
Since we are multiplying all these factors together, the highest power of x in the entire expression will be the maximum of the powers of x in each factor, which is 1.
Therefore, the correct answer is D) 0, as there is no power of x higher than 1 in the expression.