however

[joham]

.---No one believes what you tokd us yesterday.It's too surpring.
----_____,that's the truth.
A.Then B.Though C.So D.However

I ran across this question with the given answer "However" on a forum. But I thought the answer should be 'But that's the truth' and I also thought 'however' should not be used to begin a reply in such dialogues. Am I right?

Could I ask native English teachers to help me? Thank you very much.

[Super Sonic]

But = However in many contexts.

[David L.]

Single speaker:
"Nobody believes what I said. However, that's the truth, plain and simple."
"I can understand that it sounds incredible. However,..."

In the instance you give, somebody is replying to the speaker, and "however' would not be used.
Possibilities:
He: 'No one believes what you told us yesterday. It's too surprising.
She: 'It's the truth though'
He: 'No one believes what you told us yesterday. It's too surprising.'
She: 'Maybe, but it's the truth.'