Final answer:
The year 1423 marked the start of a war with Milan, a city-state with naval strength but weaker ground forces. Florence prepared for this war, perceiving it as a struggle to maintain its liberty and freedom in the face of Milanese threats.
Step-by-step explanation:
The year 1423 marked the first war with Milan, an Italian city-state that had a strong navy but struggled on the ground. The answer to the question is d) Milan. At that time, Florence, another Italian city-state, prepared for war against the son of the Duke of Milan who had threatened them earlier. The military conflicts of the era were set against a backdrop of the rise of city-states in Northern and Central Italy, and the prosperity they enjoyed due to trade and the decline of feudalism.
Florence saw itself as an heir to the Ancient Roman Republic, embodying freedom and liberty. The Florentine citizens interpreted their survival against invaders as signs of divine favor. Meanwhile, Venice was a wealthy maritime republic with a stable government, but it was not the city-state involved in the conflict referenced in 1423, which was solely between Florence and Milan.