See the list of IFoS(IFS) Optional Subjects

(i) Agriculture
(ii) Agricultural Engineering
(iii) Animal Husbandry & Veterinary Science
(iv) Botany
(v) Chemistry
(vi) Chemical Engineering
(vii) Civil Engineering
(viii) Forestry
(ix) Geology
(x) Mathematics
(xi) Mechanical Engineering
(xii) Physics
(xiii) Statistics
(xiv) Zoology

Provided that the candidates will not be allowed to offer the following combination of subjects :
(a) Agriculture and Agricultural Engg.
(b) Agriculture and Animal Husbandry & Veterinary Science.
(c) Agriculture and Forestry.
(d) Chemistry and Chemical Engg.
(e) Mathematics and Statistics.
(f) Of the Engineering subjects viz. Agricultural Engineering, Chemical Engineering, Civil Engineering and Mechanical Engineering—not more than one subject; What are the highest marks in Maths Optional for UPSC IAS?”