As you want to get the details that subject which must be choosen after 12th science so as to become an IAS officer (Scored 84.9% in 10th) so here is the information of the same for you:

There is no specific rule to select the subjects in your 12th or Degree you can just go with your interest

Here for your reference I am giving you the eligibility criteria of IAS exam:

Nationality:
A candidate must be either :—
a citizen of India, or
a subject of Nepal, or
a subject of Bhutan, or
a Tibetan refugee who came over to India before 1st January, 1962 with the intention of permanently settling in India, or
a person of Indian origin who has migrated from Pakistan, Burma, Sri Lanka, East African countries of Kenya, Uganda, the United Republic of Tanzania, Zambia, Malawi, Zaire, Ethiopia and Vietnam with the intention of permanently settling in India.

Age Limit:
Candidates must have attained the age of 21 years and must not have attained the age of 30 years as on 1st August of the year of examination.

Minimum Educational Qualifications:
Candidates must have passed graduation from any of Universities incorporated by an Act of the Central or State Legislature in India or other educational institutions established by an Act of Parliament or declared to be deemed as a University Under Section-3 of the University Grants Commission Act, 1956, or possess an equivalent qualification.

Number of Attempts:
Every candidate appearing at the examination who is otherwise eligible, shall be permitted four attempts at the examination.
This restriction on the number of attempts will not apply in the case of Scheduled Castes and Scheduled Tribes candidates who are otherwise eligible.
The number of attempts permissible to candidates belonging to Other Backward Classes, who are otherwise eligible shall be seven.
The relaxation will be available to the candidates who are eligible to avail of reservation applicable to such candidates.

Contact Details:
Union Public Service Commission
UPSC,
Man Singh Road Area,
New Delhi,
Delhi 110069
India

Map Location: