I ended up with:

$\displaystyle a = -3c + \frac{2}{7}d + \frac{46}{7}$

$\displaystyle b = \frac{8}{7}d + \frac{2}{7}d$

$\displaystyle d = \frac{13}{11}$

I chose option A and option D as being true... but got it wrong. Could someone possibly take a look please?