你首先要了解不定代词much的一种惯用法。

be not much of a +单数名词

be too much of a +单数名词

be as much of a +单数名词+as比较分句。

在这种结构中，much of a +单数名词 =a good/big/great+单数名词。例如：

I‘m not much of a dancer. 我不是一个好舞者。

It's too much of a risk. 这件事风险太大。

He is as much of a dancer as you are. 他和你一样都擅长跳舞。

由此你就可以知道D是唯一正确选项。

如果要用more...than...结构取代as much...as...结构，more就应该在of a +单数名词之前。如果B项改为：The experiment was more of a success than we had anticipated. 也就正确了。但这里少了介词of，所以为错误。