Yes, you can appear in the Civil Engineering optional subject exam if you have a BTech degree in Civil Engineering. The Civil Engineering optional subject is one of the many optional subjects offered in the UPSC Civil Services Examination. Here is a detailed explanation of the eligibility criteria and the process to appear in the exam:

Eligibility Criteria:
To appear in the Civil Engineering optional subject exam, you need to meet the following eligibility criteria:

1. Educational Qualification: You should have a BTech degree in Civil Engineering from a recognized university. The degree should be equivalent to the qualifying degree prescribed by the UPSC.

2. Age Limit: You must be between 21 to 32 years of age as of 1st August of the examination year. However, certain age relaxations are available for candidates belonging to reserved categories.

3. Nationality: You should be a citizen of India, a subject of Nepal, a subject of Bhutan, or a Tibetan refugee who came to India before 1st January 1962 with the intention of permanently settling in the country.

Process to appear in the exam:
To appear in the Civil Engineering optional subject exam, you need to follow these steps:

1. Fill the UPSC Civil Services Examination application form: Visit the official website of the UPSC and fill the online application form for the Civil Services Examination. Provide all the required details, including your educational qualifications, personal information, and choice of optional subject.

2. Pay the application fee: After filling the application form, you need to pay the prescribed application fee. The fee can be paid online through various modes such as net banking, credit card, or debit card.

3. Download the admit card: Once the application process is complete, the UPSC will release the admit cards. Download your admit card from the UPSC website using your registration number and date of birth.

4. Prepare for the examination: Start your preparation for the Civil Engineering optional subject exam by referring to the recommended books, previous years' question papers, and study materials. Joining a coaching institute or taking online courses can also help in your preparation.

5. Take the examination: On the scheduled date, appear for the Civil Engineering optional subject exam. The exam consists of two papers, each of 250 marks. Each paper is of three hours duration.

6. Result and further selection process: After the examination, the UPSC will release the result. If you qualify the Civil Engineering optional subject exam, you will be eligible for the next stage of the selection process, which includes the mains exam, interview, and final selection.

Conclusion:
Having a BTech degree in Civil Engineering makes you eligible to appear in the Civil Engineering optional subject exam of the UPSC Civil Services Examination. Make sure to fulfill the eligibility criteria and follow the application process to appear in the exam. Prepare well for the examination and give your best to increase your chances of success.