To determine the most appropriate combination of safety signs for a work area containing flammable chemicals and moving machinery, we need to understand the purpose and meaning of different safety signs:

• Danger Signs: These signs indicate a high level of risk, where severe injury or death might occur. They are used in areas where immediate hazard exists and caution should be exercised.
• Warning Signs: These signs indicate a hazard level lower than that of danger signs but still can cause significant harm or injury.
• Caution Signs: These signs are used for alerting about potential hazards that may result in minor or moderate injury.

Given this information:

1. Chemicals: For flammable chemicals, it is crucial to use signs that represent a high risk because of the potential for explosive or highly dangerous reactions, demanding the use of Danger Signs.
2. Machinery: While moving machinery poses significant risks of injury, it is generally lower than that of explosive chemicals. Thus, Warning Signs would be appropriate to caution about moving parts and machinery operation.

Based on the above analysis, the correct option is: Danger signs for chemicals and warning signs for machinery

Let's evaluate the provided options:

• Caution signs for chemicals and warning signs for exits: This option misunderstands the severity of chemical hazards and is primarily focused on exit hazards.
• Danger signs for machinery and caution signs for chemicals: This reverses the necessary levels of caution, underrating the risk from chemicals.
• Warning signs for chemicals and caution signs for machinery: This underestimates the severe risk from chemicals and moderately overstates the risk from machinery.
• Danger signs for chemicals and warning signs for machinery: This matches the appropriate level of alertness required for each hazard type.

Therefore, the correct answer is to use Danger signs for chemicals and Warning signs for machinery due to the nature and level of risk portrayed by both hazards.