This is educational app. Some products having 3 or 4 buttons, example ( Buy e- book, Rental e-book, Buy Chapters, Buy Combo) What is the Best way to show this button in app.
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Four types of buy button? I think we've found the problem. :)
Think about having just one Buy button, and providing buy options: EBook, Rent, Buy Chapters, and Buy Combo.
The analog would be buying shoes online: There's just one Buy (or Add to Cart) button but before you can do that you have to specify size, width, color, etc.
In addition to Ken's answer (select what you want, then "Buy" it), I think you would benefit if you made the various options more "dynamic"...
If you select to buy or rent the e-book, then (I presume) you get all chapters. If so, then by all means list the chapters if you think it useful, but you should disable the ability to select the chapters1.
If, as in your example, the user is in "chapter mode" but has not selected any chapter, the "Buy" button should be disabled (there's nothing to buy!)
If "Rent e-book" doesn't mean "the whole thing", and you can, in fact, rent individual chapters, then instead of three buttons/options for "Buy e-book", "Rent e-book" and "Buy chapters", you probably want two independent selectors:
(I've ignored "Buy combo" because I don't know what it means... fit it into the above as appropriate).
The reason: the user might have started in "chapter" mode and selected a number of chapters. Presumably, at some point it will become cheaper to get the whole e-book instead of individual chapters (or, at least, the difference becomes small enough to justify getting the whole book instead of only parts). Near that point2, the user might click "Buy e-book" to see how much it would be. If they don't like the price, and return to "chapter mode", it would be nice if you restore their previous selection.
2 For bonus points (and possible "up-sell"), you could monitor how many chapters the user has selected and display a message along the lines of "For only £xx.xx more you can buy the whole e-book!".