The presence of a comma after the independent clause
According to a recent survey, the establishment of natural parks to preserve existing plant and animal life in Colorado is still a priority of a majority of residents shows that a subordinate clause is required. We can, therefore, eliminate A and B.
The verb in the preceding clause is
is. The right tense for the second clause has to be the past tense of
is, which is
was. This is because we are comparing a recent survey result of a group of residents to the result of a survey conducted in the past for a different group. We can, therefore, eliminate C and D because of the use of verbs
did and
have, which are plural.
The subject of the first clause is
establishment, which is singular. So the pronoun
it in option E agrees in number with the antecedent
establishment in the preceding clause. In addition, the verb
was is in the right tense for an action that took place in the past.
The correct answer is therefore
option E in my view.