💡 Detailed Explanation
The correct choice is 'なっ' (forming '安くなったら' - if it becomes cheap). The conditional '〜たら' form of I-adjectives is created by changing the final 'い' to 'かったら'. Thus, '安い' becomes '安かったら' (yasu + kat + tara). Since the prompt is '安く（ ）たら', it must be '安くなったら' (if it becomes cheap) using the verb 'なる' (to become) in its conditional form 'なったら'. Therefore, 'なっ' is the correct drop-in for '安く（なっ）たら'!