Which of the following statements is true in the context of grammar?

To determine which statement is true in the context of grammar, let's analyze each option systematically:

1. Grammar provides a set of rules that should be memorized before being followed:
While grammar does involve rules, modern language acquisition theories suggest that memorizing rules in isolation is not the most effective way to learn or use a language. Language is often acquired naturally through usage, context, and communication, rather than rote memorization. Therefore, this statement is not entirely accurate.

2. Grammar is a system of language which helps in polishing our speaking and writing skills:
Grammar acts as the structural framework of any language. It defines how words are structured, combined, and interpreted. A strong understanding of grammar allows speakers and writers to express their thoughts clearly, precisely, and effectively, thereby directly polishing both spoken and written communication. This statement is accurate and universally accepted.

3. Grammar has no role in the learning of a foreign language:
This statement is incorrect. Grammar plays a crucial role in learning a foreign language because it provides the structure and rules that allow a learner to construct correct sentences and understand the relationships between words in a new language.

4. Knowledge of grammar helps acquire fluency in speech:
Fluency is primarily about the flow, speed, and ease of communication, which is developed through practice, exposure, and confidence. While grammar knowledge ensures accuracy in speech, having extensive theoretical knowledge of grammar rules does not automatically guarantee fluency and can sometimes even make learners hesitant as they overthink the rules. Thus, statement 2 is a more comprehensive and accurate description of grammar's primary role.

Conclusion:
The most accurate statement is that grammar is a system of language that serves to refine and polish our speaking and writing skills.