Concept:
The Chief Ministers of Andhra Pradesh are arranged in chronological order based on their term of office. To identify the 6th Chief Minister, we count the sequence historically.
• 1st Chief Minister -- Neelam Sanjiva Reddy
• 2nd Chief Minister -- Damodaram Sanjivayya
• 3rd Chief Minister -- Neelam Sanjiva Reddy
• 4th Chief Minister -- Kasu Brahmananda Reddy
• 5th Chief Minister -- P. V. Narasimha Rao
• 6th Chief Minister -- Marri Chenna Reddy
Step 1: Count the Chief Ministers in sequence.
By checking the order of office:
Marri Chenna Reddy was the 6th Chief Minister of United Andhra Pradesh.
Hence, the correct answer is:
\[
\boxed{\text{Marri Chenna Reddy}}
\]