Before I went downstairs I had prepared myself very carefully ____I must say.

Before I went downstairs I had prepared myself very carefully ____I must say.

b prepare for是固定词组，又因为是我说的东西，所以用what。

B
请看来自牛津辞典的注释，第二条：
prepare
verb
1 [VN] ~ sth/sb (for sb/sth) to make sth or sb ready to be used or to do sth: to prepare a report / the accounts ◆ A hotel room is being prepared for them. ◆ The college prepares students for a career in business.
2 ~ (yourself) (for sth) to make yourself ready to do sth or for sth that you expect to happen: [V] I had no time to prepare. ◆ The whole class is working hard preparing for the exams. ◆ [VN] The police are preparing themselves for trouble at the demonstration. ◆ I had been preparing myself for this moment. ◆ Her condition is not improving as we hoped. You must prepare yourselves for the worst. ◆ [V to inf] I was preparing to leave. ◆ [VN to inf] The troops prepared themselves to go into battle.
3 [VN] to make food ready to be eaten: He was in the kitchen preparing lunch.
4 [VN] ~ sth (from sth) to make a medicine or chemical substance, for example by mixing other substances together: remedies prepared from herbal extracts

for what.