Which is the following is correct?

a) <form ... onSubmit="return checkData()">

b) <form ... onSubmit="return checkData();">

c) <form ... onSubmit="checkData()">

d) <form ... onSubmit="checkData();">

....where checkData is a JavaScript function that returns a true or false 
value.

It seems that Internet Explorer accepts all of the above, but I'm not 
taking that to mean that all are correct.

I've attempted to search the W3 web site on the terms form and onsubmit, 
but the documentation refers to the attribute value as being an intrinsic 
event (without giving any examples).  The archived mailing lists contain 
messages which demonstrate all of the options that I've provided above.  I 
gave up after the forth page of search results because of the number of 
conflicting examples.

Does anyone know which is correct?

Thanks for your precious time.