Diels Alder reaction with maleic anhydride. Reactivity of different dienes
a) The compound can rotate into a trans configuration that is not suitable for Diels Alder

b) The compound is locked into a cis configuration that is favorable. Furthermore, a carbon group is slightly electron donating

c) The compound has an electron withdrawing group.

d) What compound is that? It's compounds labelled b) and c)

Comment: Normal demand Diels Alder is enhanced by an electron withdrawing group in the dienophile and an electron donating group in the diene. Look at reaction mechanism tab (http://en.wikipedia.org/wiki/Diels%E2%80%93Alder_reaction) 

I am not sure as to why that is. I have forgotten.