Introduction:
The broken chain symbolizes the act of becoming free. This symbol has been used throughout history to represent liberation and the breaking of oppressive systems. In the context of this question, the broken chain represents the breaking of the chains of slavery and the attainment of freedom.

Explanation:
1. Chains used to fetter slaves:
The broken chain symbolizes the end of slavery and the liberation of enslaved individuals. Slavery was a system in which human beings were considered property and were forced to work without consent or payment. The breaking of the chains represents the abolition of slavery and the recognition of the inherent rights and freedom of every individual.

2. Strength lies in unity:
While the broken chain does not directly symbolize strength in unity, it can be interpreted in this way. The act of breaking free from oppression often requires collective action and unity among those who are oppressed. When people unite against a common oppressor, they can break the figurative chains that bind them and achieve freedom.

3. Royal power:
The broken chain does not symbolize royal power. Royal power is often associated with authority and control, which is contrary to the concept of breaking free from oppressive systems. The broken chain represents the opposite of royal power - it signifies the triumph of individual freedom over the control and dominance of monarchies or oppressive regimes.

4. Act of becoming free:
The correct answer to this question is option 'D' - the broken chain symbolizes the act of becoming free. When individuals are liberated from the chains of slavery, they experience a newfound sense of freedom and autonomy. The breaking of the chain represents the end of their subjugation and the beginning of a life of independence and self-determination.

Conclusion:
In conclusion, the broken chain symbolizes the act of becoming free. It represents the abolition of slavery and the attainment of freedom. While it does not directly symbolize strength in unity or royal power, it can be interpreted in those contexts. The broken chain stands as a powerful symbol of liberation and the triumph of the human spirit over oppression.