The correct option is A The pen that you gave me is very beautiful.
The basic order of words in a sentence is: subject + verb + object.
To
this basic structure, different elements can be added. For example,
noun phrases, adjective phrases, subject complements, types of clauses
and so on.
Option A: The structure in this sentence complies with the basic structure:
subject
(The pen that you gave me) + main verb (is) + subject complement
(very beautiful). "The pen that you gave me" is a noun clause that functions as the subject.
Hence A is correct.
Option B: The noun clause does not have the correct word order: 'You gave me the that pen".
Option
C: Similarly, the order of words here isn't correct either. The noun clause has been separated by the main verb 'is' and the complement 'very beautiful'.
Option D: The order is a jumble of words which do not form a coherent meaning.
Hence options B, C and D are incorrect.