The study of language is known as

The study of language is known as- Philology.

খ) Philology (Noun)
Bangla Meaning: ভাষাবিজ্ঞান; ভাষাতত্ত্ব।
English Meaning: the study of literature and of disciplines relevant to literature or to language as used in literature.

Other options:
ক) Oncology (Noun)
Bangla Meaning: ক্যান্সারবিজ্ঞান ।
English Meaning: a branch of medicine concerned with the prevention, diagnosis, treatment, and study of cancer.

গ) Theology (Noun)
Bangla Meaning: ধর্মতত্ত্ব; ঈশ্বরের প্রকৃতি এবং ধর্মীয় বিশ্বাসের ভিত্তি সম্পর্কিত জ্ঞান শাখা।
English Meaning: the study of religious faith, practice, and experience.

ঘ) Cosmology (Noun)
Bangla Meaning: মহাবিশ্ব সম্পর্কিত বিজ্ঞান।
English Meaning: a branch of metaphysics that deals with the nature of the universe.

Source: Merriam-Webster Dictionary, Accessible Dictionary.