"Leu" Refers to a Gene That Governs a Cell's Ability

"Leu" refers to a gene that governs a cell's ability to synthesize leucine from inorganic precursors. What does the designation "leu^-" indicate?

A) that the bacterium is biologically inactive
B) that the allele is the mutant allele
C) that the bacterium is biologically active
D) that the allele is normal (i.e., wild type)