The correct option is C that
A relative pronoun has two functions, it refers to a noun that has been mentioned before, and it connects two sentences together.
In the given sentence, the relative pronoun isn't present. But, it is implied. There are many instances where the relative pronoun 'that' is omitted, because the sentence still means the same with or without it. In such a situation, it is said that the word 'that' is implied.
The given sentence is one such situation.
The sentences:
'Large and wooden were the boxes he stacked up.' and
'Large and wooden were the boxes that he stacked up.' mean the same.
Hence option C is correct.
'Boxes' is a noun, not a pronoun, 'he' is the subject pronoun, not a relative pronoun and 'the' is an article. Hence options A, B and D are incorrect.