(a) y''' + y' - xy = x
(b) y'' - y' = xy2
(c)x/y = y'
(d)(xy')' = cos x
if you review your text, you can easily find what the order of a DE is. (think of the order of a polynomial).
As for linear, a DE is linear just as a polynomial is linear.
So, take your best guess, and then type your DE in at wolframalpha.com and it will tell you whatcha got. For example,
http://www.wolframalpha.com/input/?i=y%27%27%27+%2B+y%27+-+xy+%3D+x