Step 1: Understand Goods and Services Tax (GST).
GST is a comprehensive, indirect tax levied on the supply of goods and services in India. It subsumed almost all indirect taxes except for a few state taxes, creating a unified common market.
Step 2: Know that GST required a Constitutional Amendment.
Since taxation powers are divided between the Union and States, introducing a single tax like GST required a constitutional amendment to give both levels of government the power to legislate on it.
Step 3: Identify the specific amendment.
The amendment that introduced GST was the Constitution (One Hundred and First Amendment) Act, 2016. This amendment inserted new articles like 246A (which gives power to both Union and States to levy GST) and 279A (for the creation of the GST Council).