Read the following sentence carefully and select the pair of words from the given choices that appropriately fills the two blanks to render the statement grammatically correct and meaningful.

Drafting a comprehensive report is _____________ demanding, but even _____________ it is an arduous task, researchers must complete it regularly.

The correct option is ceaselessly, so.

Let us analyze the sentence structure and the meaning of the words to see how the chosen pair fits appropriately into the blanks:

"Drafting a comprehensive report is ceaselessly demanding, but even so it is an arduous task, researchers must complete it regularly."

Step-by-Step Breakdown:

1. First Blank: The word required here modifies the adjective demanding. The adverb ceaselessly means continuously or without pause. Using it here emphasizes that drafting a comprehensive report requires persistent, uninterrupted effort.

2. Second Blank: The second blank works in combination with "even" to form the transitional phrase even so, which means "in spite of that" or "nevertheless". It logically connects the assertion that the task is arduous with the fact that researchers must complete it regularly regardless of its difficulty.

Therefore, the word pair ceaselessly, so completes the sentence with proper grammatical flow and clear meaning.