Read each sentence to find out whether there is any grammatical error in it.…

Concept: In sentence-correction / error-spotting questions, each labelled segment of the sentence is checked independently against ordinary rules of English usage - articles, prepositions, tense, and agreement are the usual fault lines. Certain scripture/canonical-text names are conventionally referred to with the definite article 'the' as a fixed part of how the name is idiomatically used (the Bible, the Quran, the Gita) - unlike most other unique proper names (people, places), which usually take no article at all - so dropping 'the' before such a title is a recognisable error.

Application: Checking each segment in turn - by established convention this scripture is referred to as "the Bible"; the definite article is a fixed part of the idiomatic name, so bare "Bible" here is non-standard and needs "the" before it. "can be" uses the modal "can" with the passive infinitive "be regarded" - a standard, correctly formed construction expressing possibility, so no fault here. "regarded as" is the standard collocation linking a subject to how it is described, so no fault here either. "a great book" has the indefinite article "a" immediately followed by the noun phrase "great book", forming a complete, well-formed indefinite noun phrase, so this segment is fine too. Exactly one segment carries a genuine error - the missing definite article before this conventionally-titled work - so that segment is the answer.

Cross-check: Rewriting the sentence with the fix applied gives "The Bible can be regarded as a great book," which reads as fully grammatical modern English - confirming the identified segment was indeed the flaw and that no other segment needed a change.