Step 1: River valley projects.
South India has several major river projects. Tungabhadra is the biggest in the region.
Step 2: Analyzing the options.
(A) Tungabhadra: Correct — biggest river valley project in South India.
(B) Sharavati: Known for Jog Falls hydropower, but not the biggest.
(C) Chambal: Located in Central India, not South.
(D) Hirakud: Located in Odisha (Eastern India).
Step 3: Conclusion.
Thus, the biggest river valley project in South India is the Tungabhadra project.