Choose the antonym of 'Mendacious' -

Mendacious (adjective)

English Meaning: Not telling the truth; lying.
Bangla Meaning: (আনুষ্ঠানিক) মিথ্যা; মিথ্যাবাদী: mendacious reports.

Synonyms: Untruthful (মিথ্যাবাদী), Dishonest (অসাধু), Deceitful (প্রতারণাপূর্ণ), Lying (মিথ্যা), Fraudulent (ধোঁকাবাজি)। 
Antonyms: Truthful (সত্যনিষ্ঠ), Honest (সৎ), Veracious (সত্যনিষ্ঠ), Frank (নির্ভীক), Sincere (একনিষ্ঠ)। 

Other Forms:
Mendaciously (adverb) অসত্যভাবে।
Mendacity (noun) (আনুষ্ঠানিক) [uncountable noun] মিথ্যা; [countable noun] (plural mendacities) অসত্য বিবরণ।

Example Sentence: 
1. That promise has been revealed as mendacious nonsense.
2. The newspaper story was mendacious and hurtful.

Example Sentence: ‘That promise has been revealed as mendacious nonsense.’

other option:
Continent (সংযমী)

Reference: Website Lecture.