I think as dragonchild
Option B is "more readable" for me, but there are situations in which I use option C.
I use option C if expr is long or complicated.
I use option C also if there is one possibility to need one else now or later.
I think I am a little fussy, cause I don't like

unless (a()){
 b();
}
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I prefer

b() unless a();
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because I find that really more readable.
Err, I think

not use 'option_A' unless golf() or advanced_programmer();
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:-)
Hopes