Choose the most appropriate option to fill in the blanks in the given…

The correct answer is Option (3) → not only, but also

Correlative conjunctions:

• Correlative conjunctions work in pairs and join two equal parts of a sentence: either ... or, neither ... nor, not only ... but also, both ... and.
• Not only ... but also adds the second item to the first: both are included.
• This sentence: a complete critical analysis needs attention to the form and to the content, so both must be included.
• Deciding clue: "a complete critical analysis".
• Complete sentence: While reading a literary text, students must pay attention not only to the form but also to the content to ensure a complete critical analysis.

Why the Other Options are Incorrect?

Option 1: either, or — Incorrect. Either ... or offers a choice of one of the two. A complete analysis needs both form and content.

• First blank "either": "pay attention either to the form" begins a choice of one.
  Example: You can sit on either side.
• Second blank "or": "or to the content" ends the choice, so one of the two would be left out.
  Example: Is it Monday or Tuesday today?

Option 2: neither, nor — Incorrect. Neither ... nor rules out both. Students would pay attention to nothing, the opposite of a complete analysis.

• First blank "neither": it denies the form, which clashes with "must pay attention".
  Example: Neither answer is correct.
• Second blank "nor": it denies the content as well, so nothing is studied.
  Example: He did not call, nor did he write.

Option 4: such, as — Incorrect. Such ... as is used for examples or comparison (such books as these). "Pay attention such to the form as to the content" is not English.

• First blank "such": it must come before a noun (such a crowd). It cannot stand before "to the form".
  Example: I have never seen such a crowd.
• Second blank "as": it compares or names a role. "As to the content" does not complete the sense.
  Example: She works as a nurse.

Contrast: not only ... but also includes both. Either ... or picks one of the two, and neither ... nor rules out both.