challenge的形容詞形式是什麼？

例句：

Bill was stimulated by the challenge.

比爾受到該挑戰的激勵。

She beat off a challenge to her leadership.

她戰勝了對她的`領導地位的挑戰。

I hope that he proves equal to the challenge.

我希望他最後能應付這一挑戰。