Final answer:
The correct phrase to complete the given sentence in formal English is 'c. For its superior quality'. The other options are either grammatically incorrect or redundant in the context of the sentence. 'For its superior quality' provides a clear and concise reason that connects properly to the main clause.
Step-by-step explanation:
The sentence in question should be completed using formal English. The correct choice for completing the sentence 'This product will succeed because ______.' with formal English is 'c. For its superior quality'.
Option a, 'Of its superior quality', is not grammatically correct in this context, as it lacks a preposition that connects the phrase properly to the main clause. Option b, 'Because of superior quality', is redundant since the word 'because' is already present in the main clause and should not be repeated. Option d, 'Being of superior quality', is a participle phrase that does not correctly fit the structure of the sentence.
In contrast, option c, 'For its superior quality', correctly uses 'for' as a conjunction to provide the reason for the product's success, hence completing the sentence in a manner that is both grammatically correct and formally structured.