Which of the following rules for writing SI symbols is incorrect?

The reason why separating numbers into blocks of three digits each using commas is incorrect in the context of writing SI symbols is rooted in the established conventions for scientific notation and the SI (International System of Units) system. In scientific and technical writing, it is standard to use a space to separate groups of three digits when writing large numbers. Commas are not used to group digits; instead, they can lead to confusion, especially in various cultures where commas may serve as decimal points.

For instance, in expressing a value like one thousand, it can be clearly indicated as 1 000. This method of spacing enhances clarity and follows the international standard, which seeks consistency and minimizes potential misinterpretation. Therefore, option C does not adhere to the preferred practices of writing SI symbols, making it the incorrect choice in this context.

The other rules provided are valid according to standard SI conventions. Leaving a space between a number and its symbol maintains clarity, using decimals instead of fractions aligns with mathematical precision and ease of understanding, and employing the multiplication sign when expressing mathematical operations follows commonly accepted notation conventions.