UPSC Civil Services Examination is considered as one of the toughest examinations in India. The examination is conducted in three stages - Prelims, Mains, and Interview. The Prelims examination consists of two papers - General Studies Paper 1 and General Studies Paper 2 commonly known as CSAT.

The correct answer to the given question is option 'A'. Let's discuss why option 'A' is the correct answer.

General Studies Paper 1 is the first paper of the Prelims examination. This paper is a test of the candidate's knowledge of current events, history, geography, Indian polity, economics, and general science. The paper consists of 100 multiple choice questions carrying 2 marks each.

Now, let's discuss why option 'B', 'C', and 'D' are not the correct answers.

Option 'B' - General Studies Paper 2, also known as CSAT, is the second paper of the Prelims examination. This paper is a test of the candidate's aptitude and reasoning skills. The paper consists of 80 multiple choice questions carrying 2.5 marks each.

Option 'C' - Mains examination consists of nine papers in total - two qualifying papers (English and any Indian language), one essay paper, four General Studies papers, and two optional papers.

Option 'D' - Interview or Personality Test is the final stage of the UPSC Civil Services Examination. Candidates who qualify the Mains examination are called for the Interview round. The Interview round is conducted to assess the candidate's overall personality, mental alertness, and critical thinking ability.

In conclusion, General Studies Paper 1 is the correct answer to the given question as it is the first paper of the Prelims examination.