A hypothesis is a supposition put forward in explanation of observed facts.
Is this sentence wrong??and if it is then why?
In a grammar book the right form of this sentence is given by this way - "A hypothesis is a supposition put forward to explain observed facts." or "A hypothesis is a supposition put forward as an explanation of observed facts."
There is nothing wrong with this sentence. It is both grammatical and idiomatic in even the most formal writing. Here are some examples:
Popular culture examples of the usage of the language of deadly sin demonstrate the continued usage of this language but offer little in explanation of why these associations still exist. — Hoverd, W, ‘Deadly Sin’, in Medicine, Religion, and the Body (International Studies in Religion and Society), ed. Coleman, E and White, K, 2009
In these accounts a past stimulus is cited in explanation of a present action. — Harré, R, Cognitive Science: a philosophical introduction, 2002.
The authors choose to discuss the mechanisms of central pain before describing its clinical features, although surely the latter must be used in explanation of the former. — Burchiel, K.J., ed. Surgical Management of Pain, 2002.
Oooof... how to explain this one...
You shouldn't say 'in explanation of', as it isn't the best way to construct the sentence.
To say 'to explain' in there is the best way to say it. In full, it means '...put forward in order to explain...' which is the meaning you want here.
To say 'as an explanation of' is also correct. Roughly, it can reworded to say the 'hypothesis' is treated 'as an explanation', or, in this sentence, 'put forward' as such.
To say 'in explanation of' has a different meaning. I cannot explain it easily (hoping someone can), but the construct would normally be used to describe a replacement:
...supposition put forward in place of / instead of observed facts.
It shouldn't be used with the meaning you are trying to apply to it, as the meaning becomes unclear.
