Concept:
The 7th Constitutional Amendment Act, 1956 was one of the major amendments that reorganized Indian states.
• Based on linguistic principle.
• Abolished Part A, B, C, D states.
• Created new state structure.
Step 1: Recall the purpose of 7th Amendment.
It was passed in:
\[
1956
\]
to implement the States Reorganisation Act.
Step 2: Understand what it changed.
It reorganized states mainly on:
Linguistic basis
to improve administration and regional identity.
Step 3: Check other options.
• Ninth Schedule -- 1st Amendment
• Voting age 18 years -- 61st Amendment
• 10% reservation (EWS) -- 103rd Amendment
So these are incorrect.
Step 4: Final conclusion.
Hence, the correct answer is:
\[
\boxed{\text{States Reorganisation on linguistic basis}}
\]