Choose the word opposite in meaning to “insidious”-

Insidious (adj)
Bengali Meaning: সূক্ষ ও ধ্বংসাত্মক, ছলনাপূর্ন
Meaning: causing harm in a way that is gradual or not easily noticed.
Synonym: treacherous, subtle, ambiguous, equivocal, crafty
Antonym: straightforward, innocuous
Source: Samsad Dictionary and Oxford Learner’s Dictionaries