What best describes the conventional spelling and writing system of a language?

The correct option is orthography, as it refers specifically to the conventional spelling and writing system of a language. Orthography encompasses the rules and conventions for representing spoken language in written form, including spelling, punctuation, and capitalization. This term captures the entire system that dictates how words are formed and represented in writing, making it the most suitable choice in the context of spelling and writing conventions.

Other concepts, while related to language and literacy, do not define the spelling and writing system directly. For example, graphophonics focuses on the relationship between letters and sounds, which is a critical aspect of learning to read and write, but it does not encompass the overall system of rules that make up orthography. Orthographic knowledge refers to an individual's understanding of spelling conventions and rules, which is important but is a subset of what orthography entails. Morphology deals with the structure of words and their meaningful components, which, while important in understanding language, does not specifically address the conventions of spelling and writing.