Explanation

1-x-x^n+x^(n+1) = (1-x)(1-x^n), and 1-x^n is itself divisible by 1-x, so the whole expression is divisible by (1-x)^2. Hence option (B). Since 1-x^n = (1-x)(1+x+...+x^(n-1)), the product visibly contains (1-x)^2 as a factor.