The answer would be C, structures A and B.
Haploid cells mean that the cells that only have half of the original chromosome number than other cells of the species. For example in human, the original chromosome number should be 46, but haploid cells only have 23 chromosomes. Haploid cells are only usually seen in reproductive cells, which means the cells used for sexual reproduction.
Since flowering plants reproduce sexually, the cells that are haploid are the sex cells. And for sex cells, they include pollen grains (male sex cell) and ovum (female sex cell). According to the picture, pollen can be found in structure B, the anther, and ovum can be found in A, the ovary.
So, your answer is C.
