Isomerism in amide and nitrile aldehyde

I have the following question, 
[![enter image description here](https://i.stack.imgur.com/oOebX.jpg)](https://i.stack.imgur.com/oOebX.jpg)
The given answer is c but I have trouble understanding why a would not be a correct answer. Because from my understanding in part a, one is amide functional group and the other is aldehyde with a -Nh2 nitrile group as a substituent. Then it should be a functional group isomerism too, right? 




I think choice D, *All of the above* may be the correct answer. All three pairs are structural isomers, with different functional groups, thus functional group isomers (FGIs). 


[This brief overview](http://www.chemguide.co.uk/basicorg/isomerism/structural.html) uses a carboxylic acid and ester pair as an example of FGI, which validates choice B. It also cites an aldehyde $\ce{CH3CH2(C=O)H}$ and a ketone $\ce{HCH2(C=O)CH3}$. If the methyl group is swapped out for an amino group (not called a nitrile), we have your compounds $\ce{NH2CH2(C=O)H}$ and $\ce{HCH2(C=O)NH2}$. Not only should this pair be FGIs by analogy with the aldehyde-ketone, but they satisfy the definition of functional group isomers: Structural isomers (same atoms, different connectivity) that have different functional groups. This validates choice A.


[Wikipedia](https://en.m.wikipedia.org/wiki/Structural_isomer?wprov=sfla1) cites your exact ether-alcohol pair as example of FGIs, validating choice C.


Thus, *all of the above* must be the correct answer. If you are in a class, I would ask your instructor about this problem.


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