I would ask Tom to lend us money if I _ him.A.had known B.knew C.have known D.know

D.know
if后面接原形动词

选b，因为这是一个虚拟语气的句子，意思是：如果我认识汤姆的话，我会叫他借钱给我们。事实上，我不认识叫汤姆的那个人

答案：B.
翻译：如果我认识汤姆,我会叫他给我们借钱的.
if在此引导的是虚拟条件句,从句用一般过去时,主句用would + 动原.比如：
I would certainly go if I had the time.
If were you,I would reconsider their proposal.
If he were here,everything would be all right.