Option 1 -> means deceitful, disloyal, or involving betrayal - fits with actions causing public outrage.
Option 2 -> means fascinating or absorbing - a positive quality that wouldn't cause outrage.
Option 3 -> means modest or not pretentious - a positive trait incompatible with causing outrage.
Option 4 -> means innocent or naive - doesn't align with actions deserving public condemnation.
Hence, Option 1: treacherous -> The context requires a word describing negative, deceitful actions that were hidden and, when revealed, provoked public anger. "Treacherous" perfectly captures the sense of betrayal and dishonesty that would lead to widespread outrage when exposed -> correct