this bike is ______,if not better than,that one

绝对选A，if not better than在句中是插入语,其典型标志是用两个逗号隔开,起补充说明作用.完整的句子应该是this bike is as good as that one.

A (as +adj.+as , if not better than是插入语

选A，先把if not better than去掉。this bike is as good as that one.
这是英文中常出现的插入语句型，遇到了就用这种方法做就可以了。

如果你选D,你把if not better than去掉再读来试试，很明显不通顺。as+adj.+as是固定搭配哈，这里是插入语。