Hello! Is there some kind of shortcut for this type of question? Thank you soo much in advance 
Hi!
CnH2nO is the general formula for an aldehyde, and D doesn’t fit this due to having extra Hydrogens.
A and B do not satisfy them either as they have extra oxygen in them but this could be due to a hydroxyl group which doesn’t make it not an aldehyde as they can still have aldehyde groups attached.
C6H12O2: H2C(OH)-CH2-CH2-CH2-CH2-HC=O so, it do exists and have an aldehyde group.
C5H10O2: H2C(OH)-CH2-CH2-CH2-HC=O option B also can have an aldehyde group.
Answer is thus D.
Hope it helps:)
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thank you so much Ujjwal, you are a genious 
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