Syntax comparison for function call

I have the following ideas for function call syntax
(the function sum() gets two integers as named parameters a and b and returns int):

a) c = sum.a( 42 ).b( 42 )

b) c = sum( a:42 b:42 )

Which one would you prefer and why?
In the given language there are only named parameters, all of which are required
in the call in the order they are defined.