Originally Posted by hoangkha
You should have wondered about (C) as well, because (C) must be in the simple past tense. Only this is gramatically correct:
I would have gone with him to Washington, except that I had no time.
The comma between the independent clause at the left and the dependent clause at the right is mandatory. Also, it is grammatically incorrect to omit the that from the subordinating conjuction except that.
You would do well to know that something in that construction is omitted. Let's regress from the most verbose version until we reach the above construction.
1. I didn't go with him to Washington, but I would have gone with him to Washington if I had had the time, except that I had no time
2. I would have gone with him to Washington if I had had the time, except that I had no time.
3. I would have gone with him to Washington, except that I had no time.
All three of those sentences have the same meaning, and contain the same amount of it, with only the third sentence having the usual, regular construction.
You also asked about unless, another subordinating conjunction used for expressing conditions. No, you cannot use it in your sentence. Here's the difference between unless and except that:
INDEPENDENT CLAUSE A + UNLESS + DEPENDENT CLAUSE B
= A keeps on holding, and the only way it no longer holds is on the condition that B holds
= "unless" means "but the opposite is the case if"
= "unless" means "except if"
I will love her forever unless she betrays me.
Don't come unless I telephone.
The directors have a meeting every Friday, unless there is nothing to discuss.
This dress is definitely black, unless it's indigo.
INDEPENDENT CLAUSE A + EXCEPT THAT + DEPENDENT CLAUSE B
= A almost holds, but it doesn't, because the condition B is met
= "except that" means "were it not true that"
I'm just like you, except that I'm much better with kids.
I have nothing to regret, except that I might've unintentionally hurt somebody.
A good time was had by all, except that three kids got into a fight.
Socrates didn't know anything, except that he knew that.