How can you tell "that"you are a good rider? = 你怎么能说你是个好骑手呢?
“that”引导宾语从句.

How can you tell "weather" you are a good rider? = 你怎么能说你是否是个好骑手呢?
"weather" = 是否,从语法上没错,但是句意不通.

所以,选A.