Please help! 20 Points!

1. The Mason-Dixon line, which is perhaps the most famous border ever established by surveying methods, has been designated a National Historic Landmark.

Which of the following is the main clause in the sentence?

A. The Mason-Dixon line
B. has been designated a National Historic Landmark
C. The Mason-Dixon line has been designated a National Historic Landmark
D. which is perhaps the most famous border ever established by surveying methods

2. The Mason-Dixon line, which is perhaps the most famous border ever established by surveying methods, has been designated a National Historic Landmark.

Which of the following is the adjective clause in the sentence?

A. The Mason-Dixon line
B. which is perhaps the most famous border ever established
C. by surveying methods
D. a National Historic Landmark

3. The Mason-Dixon line, which is perhaps the most famous border ever established by surveying methods, has been designated a National Historic Landmark.

Which of the following are the two participles in the sentence?

A. "Mason-Dixon line" and "National Historic Landmark"
B. "is perhaps" and "established by surveying methods"
C. "the most famous border" and "surveying methods"
D. "established" and "has been designated"

A main clause also known as independent clause contains a subject and a verb with a complete express of thought. For example in the given -paragraph "Mason-Dixon is the subject and "designated" is the verb so it qualifies as main clause without this sentence would be incomplete.
An adjective clause is a type of clause the works to describe noun in the sentence. In the example "which is perhaps the most famous border ever established" explains the subject "Mason-Dixon line" and hence its the adjective clause.
A participle is a word formed from a verb it could be either past or present in the given example "is perhaps" and "established by surveying methods" qualify as participles.