Question

Pollen grains are produced by
A. Leaves
B. Stamens
C. Gynoecium
D. Pistil

Answer 1

Pollen grains are a mass of microspores in a seed plant appearing usually as fine dust. The pollen grain is a minute body, of varying shape and structure, formed in the male structures of seed-bearing plants and transported by various means (wind, water, insects, etc.) to the female structures, where fertilization occurs.

Complete answer:
In angiosperms, pollen is produced by the anthers of the stamens in flowers. Pollen consists of one or more vegetative cells and a reproductive cell. In angiosperms and certain gymnosperms, the vegetative cell forms the pollen tube that grows to meet the unfertilized ovules, and the reproductive cell is the source of the sperm. Most pollen grains consist of three distinct parts. The central cytoplasmic part is the source of nuclei responsible for fertilization. The other parts constituting the wall of the grain are an inner layer, the intine, and an outer layer, the exine.
The intine consists, at least in part, of cellulose or hemicellulose. The outer and most durable layer, the exine, is very resistant to disintegration; treatment with intense heat, strong acids, or strong bases has little effect upon it.
The constituents of the exine have been termed sporopollenin. The internal parts of the pollen grain are easily broken down, whereas the exine layer, and thus the general form of the pollen grain, is easily preserved in various kinds of sediments; the quality of preservation may vary with different environments.
So, the answer is B, i.e., Stamens

Note: In gymnosperms, pollen grain is formed in the microsporophyll of the micro strobili (male pollen cones). The study of pollen and spores is known as palynology. In a pollen grain, there are two male gametes.