Jawaban: b. with experience earned on the occupational specialities

Alasannya: setiap ada not only harus ada but also, dan karena pada kalimat soal not only diikuti dengan with, jadi pada but also juga harus diikuti dengan with, sehingga kalimat lengkapnya menjadi "the role of geologist varies not only with his area of specialization, but also with experience earned on the occupational specialities."