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I.e., God satisfies every one of man's desires, provided that His unfathomable wisdom regards its satisfaction as ultimately beneficial to the human being concerned: this is the meaning of the preposition min (lit., "out of", but in this context, "something out of") preceding the phrase "what you may be asking".
In this context, “humankind” refers only to those who deny Allah’s favours.
Sincere and true prayer in faith is answered by Allah. Thus He gives us everything which a wise and benevolent Providence can give.
I have tried to render the intensive forms of the Arabic by what I consider their near equivalent here: the phrase "given up to injustice and ingratitude" suggests habitual ignoring of just values and ingratitude for the innumerable gifts and favours which Allah has showered on mankind.