(c)

Let $P(x)=1+x^2+x^4+\ldots+x^{2010}$

${c}P(x)=\frac{1-x^{2012}}{1-x^2}$

$P(x)=\frac{\left(1-x^{1006}\right)\left(1+x^{1006}\right)}{(1-x)(1+x)}$$P(x)=\left(1+x^{1006}\right)\left(\frac{1-x^{503}}{1-x}\right)\left(\frac{1+x^{503}}{1+x}\right)$

$\left.P(x)=\left(1+x^{1006}\right) 1+x+x^2+x^3+\ldots+x^{502}\right)$

$\left(1-x+x^2-x^3+\ldots+x^{502}\right)$

Thus, $P(x)$ is divisible by $1+x+x^2+\ldots x^{n-1}$

If $n-1=502 \Rightarrow n=503$