This question tests knowledge of Arabic grammar, specifically the role of nouns after the particle إنَّ (inna), one of the الأحرف الناسخة (abrogating particles).
In the sentence إنَّ الصدقَ منجاةً, the grammatical structure is as follows:
إنَّ is a حرف ناسخ (abrogating particle). When إنّ enters a nominal sentence, it changes the case of the subject (المبتدأ) from nominative (مرفوع) to accusative (منصوب). This subject is then called اسم إنّ (the noun of إنّ).
الصدقَ carries the فتحة (accusative marker), indicating it is اسم إنّ منصوب.
منجاةً is the خبر إنّ (predicate of إنّ), also in the accusative as a خبر منصوب.
The word الصدق is neither a مبتدأ (which would require the nominative case and the absence of إنّ), nor a خبر (that role belongs to منجاة), nor simply اسم (a generic noun label that does not specify its grammatical function in the sentence).
Whenever إنّ or any of its sisters (أنّ, كأنّ, لكنّ, ليت, لعلّ) appear, the noun that follows directly is اسم إنّ, placed in the accusative case.
