i would say A, so u dont get situations where the chain is either too tight or too loose, and the grooves wont get damaged too badly when u are trying to move the tensioner, it would not be too hard to aqdjust the tension either.
material aluminum. it doesnt need to be very strong, so it can be and should be light.
check out the monty tensioners, they are the perfect ones imo.. i think the size of the bolt and the where the hole is on the frame is a big factor, tensioners can be smooth and still work (i.e. koxx ones)