Final answer:
The farm has 120 broilers which are chickens specifically raised for meat production, hence 120 chickens are ready for meat production.
Step-by-step explanation:
The question asks us to identify how many of the chickens on Siobhan's poultry farm are ready for meat production. When discussing poultry, the term 'broilers' specifically refers to chickens that are raised for meat production. The farm has 120 broilers, which indicates that these are the chickens intended for this purpose. Hens, chicks, and roosters generally have roles in egg production, breeding, and flock leadership, respectively. Therefore, the correct answer is that 120 chickens are ready for meat production, which correlates to option c. 120.