(c) That's wrong. This question involves the contents of a contract, and in particular, the rules that govern how the terms of a contract are proved.

It is true to say that the agreed terms of a contract can be partly written and partly oral, and that in such cases the courts will admit evidence of both written and orally agreed terms. But that is not the end of the matter, because of the parol evidence rule.

Under this rule, if there are written terms, the court will ask whether the written terms appear to be all those that are needed for a workable transaction. If so, evidence of additional orally agreed terms will not be permitted. This rule exists to give certainty to contracts that appear complete in their written form. Would the parol evidence rule by applied in the present case? The given facts can be compared to both Van den Esschert v Chapell and LG Thorne & Co v Thomas Borthwick.

In Van den Esschert's case, the court held that something obvious was missing from the written terms - some guarantee against white ants. In LG Thorne's case, the court held that nothing was obviously missing from the written terms and therefore refused to allow proof of additional oral terms. The facts in the case study appear more similar to LG Thorne v Thomas Borthwick. This is because it would be hard to argue that, without a term regarding recycled glass, this contract is incomplete. The third answer (c) is simply wrong and should be easily recognised as such. Accordingly, the best answer is (b).