"Claws" can refer to the type of evolution ("Claws (ex)") or the number of attacks (such as "two claw attacks"). With regard to evolutions, instances of the singular word "attack" being used to indicate a plural occur in phrases like "select one type of natural attack". This can be seen in both the Push and Pull evolution descriptions. As the Reach evolution doesn't state to "select one type of attack", I have to accept "pick one attack" as meaning just that: pick one attack.
The problem with this interpretation is that you open the floodgates. Why limit yourself to "claw" when "primary" or even "melee" would be better options.