minus one times the slope of the isoquant, or the marginal product of labor divided by the marginal product of capital (MPL/MPK).
minus one times the slope of the isoquant, or (?L/?K), which is equal to (MPL/MPK)
(-?K/?L) or (MPK/MPL), where MPK is the marginal product of capital and MPL is the marginal product of labor.
None of the above.