Choose the correct word from the options given below to fill in the b…

The correct answer is Option (2) → known

Third conditional with inversion:

• The third conditional talks about an unreal past: a condition that did not happen and its imagined result.
• Pattern: If + subject + had + past participle, subject + would have + past participle.
• In formal English, "if" can be dropped and had placed before the subject: "If she had known" → "Had she known".
• Clue word: "would have". This sentence: the blank comes right after "Had she", and the result clause is "she would have made sure to attend". After "had" the verb must be the past participle of "know", which is known.
• Complete sentence: Had she known the importance of this conference, she would have made sure to attend.
  Example: Had I known about the traffic, I would have left earlier.

Why the Other Options are Incorrect?

Option 1: knew — Incorrect. Knew is the simple past, not the past participle; "had knew" is ungrammatical.

Option 3: knows — Incorrect. Knows is the present tense form for he/she/it; it cannot follow "had".

Option 4: been knowing — Incorrect.

• "Had been knowing" is a past perfect continuous form.
• "Know" is a stative verb (a verb of state, not action) and is not used in continuous forms.

Contrast: after "had" the verb is always the past participle (had known); the simple past "knew" never follows "had".