Could you please tell me if he ___finish the work on time A.could B.was able to C.is able to D.would be able to ,为什么?

C
Could 用于句首表示一种委婉的语气，不表示过去时，所以排除 A,B,D,
另外，if 引导的条件状语从句 用一般现在时 表示将来。进一步确定选C.

be able to 结构，由句意：如果他准时完成作业请你告诉我。所以选c

解题之前,先来了解下can,could 和 be able to三者的区别与联系.
1.can的过去式是could,但could 除了可作can的过去式外,还可表比can更委婉的语气,因此在回答由could 表示请求的问句中,不能用could,但可用can.如：
—Could you lend me a hand?
—Of course I can.
2.在表示现在或过去的智力、体力、技能等方面的“能力”时,can,could 常可与be able to互换.如：
He could (or was able to) swim five miles when he was a boy.
3.be able to比can有更多的表现形式,如"will be able to","have (has) been able to" "had been able to" 等等.因此be able to常被用来表示can所不能表示的将来或完成的概念.如：
They will be able to run this machine on their own in three months.他们过三个月就能自己开这台机器了.
4.在表示成功地做了某事时,不用could,而用was / were able to (或managed to,succeeded in doing sth )来表示.如：
He was able to (or managed to) finish the job yesterday.
但是couldn't和wasn't / weren't able to 常可互换.如：
I couldn't / wasn't able to finish the job yesterday.
5.与感觉动词连用时,can,could亦可与be able to互换.如：
We could/were able to smell something burning in the kitchen.
选项B was able to是过去时 而题中所问对象还未成功完成某事,所以不用过去时态,不符合题意,故B可以排除.
选项D would 先了解下它的用法与意义
1.will的过去式.
例：They said it would be fine.听说天气会很好.（it will be fine的过去式）
例：I said I do my best.我表示过要尽最大的努力.
2.（在陈述语气中,表示过去将来时,表示无意识的行动或单纯将来发生的事或动作,指实际情况,一般用于间接引语中）会；将；
例：he said he not be free that night.他说他那天晚上没有空.
所以 would be able to 是过去将来时态 不符合题意
因此选C
Could you please tell me if he is able to finish the work on time
你能告诉我他是否能够按时完成这项工作吗?