How is language Arbitrary?
There is no logical relation between the sound or written word and the object. Same object have different names in different areas shows that there is no logical relation between word and object. So, language is arbitrary.
How is linguistics a Science?
Linguistics is the scientific study/ systematic study of language. In linguistics the method is applied by making observations, testing hypotheses and deriving theories. So, linguistics is a science but social science not a practical science.
What is meant by Synchronic and Diachronic study of language?
Synchronic study of language is the study of language at a fix point or present but Diachronic study of language is the study of language change or study of language through history.
How does Ferdinand de Saussure make a distinction between Langue and Parole?
According to Ferdinand de Saussure the distinction between langue and parole is that langue is the structure of language in the mind/grammar of language in mind and the parole is the speech or written language.