Explanation:

- The government has warned the traders not to hoard onions.

- Correct Answer: This option means to accumulate and store goods in large amounts, often to create artificial scarcity, which fits the context of governmental warnings to traders.

- The government has warned the traders not to combine onions.

- This option means to join or mix together. It doesn’t fit since the likely issue is with profiteering through holding back stock, not mixing onions.

- The government has warned the traders not to gross onions.

- ‘Gross’ as a verb means to earn as profit, but it doesn't match the context. The warning is not about making profit but about stockpiling.

- The government has warned traders not to bulk onions.

- This phrase doesn’t make sense since 'bulk' refers to large quantities. However, it’s not commonly used as a verb with onions or similar contexts.