B选线省略了proportion，应为a greater proportion than （proportion）in any previous election。
Ron：there's ellipsis here. the understood comparison is a repeated instance of "proportion".
in contexts in which you would repeat a noun, you don't have to include the repeated noun; you may merely imply it. this is known as ellipsis.

for instance:
this year's heavyweight champion is shorter than last year's.
here, the second half of the comparison is clearly "last year's heavyweight champion", but you don't have to say "heavyweight champion" again.

than是作为比较代词的存在，要在句子中代指一个成分，这里代指proportion。
More people hope to live in the countryside than used to 。 代主语people
Don’t give children more money than is necessary. 代主语 money
He often eats more than is good for his health.代主语food
Tom is taller than John is. 代表语tall
I have more money than you do. 代宾语money
Tom runs faster than John runs. 代状语fast
这里贴一个探讨than的用法的帖子。http://ask.yygrammar.com/q-23051.html

还有另外一个PREP例题
Only seven people this century have been killed by the great white shark, the man-eater of the movies—less than those killed by bee stings.
A movies—less than those
B movies—fewer than have been
C movies, which is less than those
D movies, a number lower than the people
E movies, fewer than the ones

正确答案B中也省略了主语people。应为 seven people have been killed by the shark —fewer than（the people） have been killed by bee stings