Final answer:
For the transportation of chlorine, a driver must stop at railroad crossings if transporting any amount that necessitates a Table 2 placard.
Step-by-step explanation:
The amount of chlorine that requires a driver to stop before railroad crossings is any amount requiring a Table 2 placard. This is because chlorine is considered a hazardous material and regulations usually mandate that vehicles transporting hazardous materials in amounts that require placarding must stop at railroad crossings to ensure safety. Therefore, option 1) An amount requiring a table 2 placard is the correct answer.