Why is tom robinson unable to use his left arm ?

a) he was born without the use of his left arm.
b) it got caught in a cotton gin
c) it was caught under a tractor when tom was a child.
d) he fell out of tree and broke it

Greetings once again, Gary :)

- Why is Tom Robinson unable to use his left arm? -

B) It got caught in a cotton gin.

(B) is correct because Reverend Sykes tells the children (Scout, Jem, and Dill) that Tom got his hand caught in a cotton gin when he was a child. 

~Hope I helped!~