Diels Alder reaction with maleic anhydride. Reactivity of different dienes

I don't understand question 6, although I took notes and the professor mentioned an explanation in class. 


Can someone please help using easier terms?


![enter image description here](https://i.stack.imgur.com/L2ACP.jpg)




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.


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