C is the answer.Split #1: began vs begun
From the context of the sentence, we need the past tense of the verb begin rather than the past participle form. Why? We are not modifying high culture with a verb-ed modifier begun. In addition, we are not forming a past perfect tense in the sentence to warrant the use of the past particle form of the verb begin. We can, therefore, eliminate A and B based on this.
Split #2: meaning: decline of high culture vs declining culture in high places vs decline in culture in high places.
The intended meaning per the original sentence is decline in/of high culture and not decline in culture in high places or declining culture in high places. Based on this, we can clearly see that D and E change the meaning of the sentence.
We are left with C as the best answer out of the lot.
Depending on one’s point of view, the proliferation of the television is either a great advance in communication or an unmistakable sign that the decline in high culture begun before the War is still continuing.
A.
decline in high culture
begun before the War is still continuing
B.
decline of high culture
begun before the War is still continuing
C.
decline of high culture
that began before the War is continuing
D.
decline in culture in high places that began before the War is continuing
E.
declining culture in high places that began before the War still continues