Conventions are

a. the use of proper grammar b. correct spelling

c. appropriate punctuation

d. all of these

The answer is D.

Explanation:

Conventions in language are a set of rules established in order to make communication easier. They are socially accepted which means that people approve them and also use them. They are concerned with every aspect of a language including sentence structure, grammar, spelling, capitalization, punctuation etc.

For example, spelling convention can be that when at the beginning of a word letter k stands before letter n, we don't read it (knight, knife ...). Or capitalization convention is that all societies agreed that every sentence should begin with a capital letter and so on.