The correct option is C the time required to assemble a Model T from a day and a half to 93 minutes
The error(s) in this sentence is mostly due to the use of incorrect idioms. The use of 'required for assembling' in place of 'required to assemble' (the verb 'required' needs to be followed by an infinitive which is why 'required to assemble' is correct) is incorrect. Next, the use of 'reduce from...to...assembling of a Model T' is also incorrect. The correct form here should be 'reduce the time... to assemble a Model T from... to...'. These errors eliminate A and E. The phrase 'time required' is preferable to the inappropriate 'time being required'. Thus, option D is much preferable to B and C. B also has a superfluous word 'down' which doesn't add anything at all to the sentence. Since 'from' is missing from C the phrase 'a day ... 93 minutes' does not really makes sense. D is the only grammatically correct, direct choice that befits the context.