I thought that he had done nothing wrong and had a clear__.

B
先看原句,意思大致是表明个人观点“他怎么怎么好”.
然后看选项,B中conscience指“良心”,且一般与clear连用.说“sb have a clear conscience”（某人问心无愧）.
而A（个性；品格）勉强可以与clear连用,但显然不符语意.
C、D更是无此用法.