Final answer:
The exact rule number for Blue Signal Readily Visible to Engineer cannot be determined from the provided options as they do not align with known standard railroad signal rule numbers.
The correct answer is none of all.
Step-by-step explanation:
The rule number for Blue Signal Readily Visible to Engineer is not specified in the provided list of possible rules, which means I cannot ascertain the correct answer based on the options (1) 5.13A, (2) 5.13B, (3) 5.13C, or (4) 5.13D. Railroad signal rules can vary by country and by the operating railroad company. In general, these rules are part of a safety system that ensures that train operations are conducted safely. The blue signal typically indicates that workers are present on, under, or between rolling equipment, requiring that equipment to not be coupled to or moved. Specific rule numbers for these signals would be found in the rule book of the particular railroad.