Note that plain usage of "such that" is considered unidiomatic.
We need to say something like "such + Noun/Adjective + that" for its usage to be correct.
For example - "the crime has risen to such levels that ..."
Let us look at the answer options.
Option A - Incorrect.
"so many people have been moving to Utah
such that" - Usage of such that is incorrect.
"such that Mormons
who were once 75 percent of the population" - Usage of restrictive clause suggests that we are referring to
ONLY those Mormons who accounted for 75% percent of the population. Implies that there were other Mormons who did not account for 75% of the population. Illogical.
"Mormons who were once 75 percent of the population
are now only accounting for half of it" - Inappropriate usage of present continuous. To talk about facts/general truths we use the simple present.
Option B - Incorrect.
"Mormons [who were]
once 75 percent of the population
are now accounting" - usage of restrictive modifier changes the meaning of the sentence.
Usage of present continuous tense is inappropriate.
Also note that the intended meaning is "
so many people have been moving to Utah
that Mormons, who were once 75 percent of the population, now only account for half" - this suggests something unusual or dramatic.
However, in this option that effect is lost - "many people have been moving to Utah". This no longer conveys the intended meaning.
Option C -
"
that many people have been moving to Utah" - the usage is incorrect.
"
such that the Mormons that were once 75 percent" - Plain "such that" is unidiomatic.
"such that the Mormons
that were once 75 percent" - We do not use "that" to refer to people. Moreover, this suggests that we are referring ONLY to those who were once 75%, not to other Mormons there.
"... are now accounting for only half of it" - We need simple present.
Option D -
"
many people have been moving to Utah" - this distorts the intended meaning by suggesting that the movement was not dramatic/remarkable situation.
"
such that the Mormons"
Usage of "such that" is unidiomatic.