Answer: The correct answer for the blank is -
Sepal, petal, stamen, and pistil.
Flower is the reproductive structure of flowering plants (angisperms). It is primarily composed of four parts, which are (from outside to innerside) -
Sepals (that together form calyx, the outermost protective whorl)
Petals (that together form corolla, the second whorl)
Stamen (the male reproductive part that together forms androecium, which is the male reproductive organ).
Stamen is further composed of anther (that produces pollen) and filament (stalk like structure, which protects anther).
Pistil (the innermost part that forms gynoecium, which is the female reproductive organ). It is composed of stigma, style, and ovary.