Final answer:
The Stamp Act Congress marked the beginning of colonial self-governance as it was the first united opposition to British taxation without representation.
Step-by-step explanation:
The Stamp Act Congress was a significant turning point because it marked the beginning of a unified political front by the colonies against the British Parliament's direct taxation. While the Stamp Act itself imposed an internal tax on various printed materials used by the colonists, the real issue was the constitutional principle of taxation without representation. The Congress, consisting of representatives from nine colonies, issued a declaration that fundamentally challenged the British authority by denying its right to tax the colonists without their consent. Hence, the correct answer to why the Stamp Act Congress was a turning point is: a) It marked the beginning of colonial self-governance.