(a) That's probably wrong. During contractual negotiations, statements might be made about particular facts, but not in the form of a promise. Such statements only become terms of the contract if they were made in circumstances from which it can reasonably be inferred that the parties intended what was said to be contractually binding. Otherwise, the statements are called "representations" and do not become part of the contract, even if they had the effect of encouraging the person to whom they were made to enter the contract.
In the present case, D stated that, in average conditions, the paint should last at least five years. However, the facts do not support an inference that the parties intended this statement to be contractually binding, particularly because C seemed only to be concerned with the price of the paint and not with its quality. D's statement is therefore likely to be a mere representation.
Note: Because representations are not terms of the contract there is no contractual liability if a representation proves untrue (a misrepresentation). However, liability for a misrepresentation may exist in tort law and under s 18, 29 and 30 of the Australian Consumer Law - see the modules 'The scope of tort law' and 'Consumer protection in Australia'.