Final answer:
The costs of production, prices of related goods, sellers' expectations, and the number of sellers are not factors that directly affect the bid/ask spread, hence, option 4 is the correct answer.
Step-by-step explanation:
The question revolves around the factors that influence the bid/ask spread. The bid/ask spread can be impacted by a variety of elements such as transaction or order costs, inventory holding costs, and trading volume. However, the factors provided in the reference, namely the costs of production, the prices of related goods in production, sellers' expectations, and the number of sellers, do not directly affect the bid/ask spread. Hence, the answer to which of the provided options is NOT a factor that affects the bid/ask spread is option 4: 'All of these factors affect the bid/ask spread', as the correct statement should reflect that none of the factors listed in the reference directly impacts the bid/ask spread.