B lymphocytes are responsible for antibody-mediated immunity. They develop in the bone marrow and, after activation by an antigen, differentiate into plasma cells that secrete antibodies. This antibody production is the hallmark of B cells. T lymphocytes mature in the thymus and handle cell-mediated responses, while macrophages and neutrophils are phagocytes and do not produce antibodies.

Molds, rusts, mildews, smuts, mushrooms, and yeasts all belong to the fungal kingdom. They share key features: they are eukaryotic and obtain nutrients by absorbing them from their surroundings (not by photosynthesis), their cell walls contain chitin, and they reproduce mainly through spores, which explains how widespread and varied they are—from single-celled yeasts to complex mushroom fruiting bodies and plant-damaging rusts and smuts. In contrast, bacteria are prokaryotes with no nucleus, algae are typically photosynthetic and often aquatic, and protozoa are single-celled eukaryotes with diverse modes of nutrition. Because the listed organisms fit these fungal characteristics, they are classified as fungi.

Having two different alleles for a gene means the genotype at that locus is heterozygous. Each person inherits one allele from each parent, so when those two copies are not the same (for example, A and a), you’re heterozygous. This is different from homozygous, where both copies are identical (AA or aa). Monosomy refers to having only one copy of a chromosome, not two alleles at a single gene. Hemizygous describes having only one allele for a gene in a diploid organism, often used for sex-linked genes in males.

Differentiating bacteria by how their cell walls take up dye is what this question tests. The Gram stain uses crystal violet and iodine to form a dye complex inside the cell wall. In Gram-positive bacteria, the thick peptidoglycan layer traps this complex, so after a brief alcohol wash they stay purple. In Gram-negative bacteria, a thinner peptidoglycan layer and an outer membrane don’t retain the dye, so they’re decolorized and then take up a counterstain (pink/red). This creates two distinct appearances that separate the two groups. Other stains target different features: Giemsa is for blood cells and certain parasites, flagella stain highlights motility structures, and capsule stain reveals a capsule around some bacteria, not Gram-type.