Final answer:
If labor and capital are perfect substitutes, the isoquants are straight lines, reflecting constant marginal rates of technical substitution between the two inputs (Option 1).
Step-by-step explanation:
If labor and capital are perfect substitutes in production, the shape that isoquants take is: isoquants are straight lines.
When labor and capital can be substituted for each other in production on a one-to-one basis without affecting the output level, firms can freely choose between all combinations of labor and capital, and the isoquants reflecting these combinations are straight lines.
These straight lines indicate constant marginal rates of technical substitution (MRTS) between labor and capital.
Hence, the correct answer is Option 1.