The haploid number of chromosomes in gymnosperms is given as 12. To determine the number of chromosomes in the root and endosperm, we need to understand the process of fertilization and the formation of these structures.

1. Fertilization in gymnosperms:
- Gymnosperms are seed-bearing plants that include conifers, cycads, and ginkgo.
- They have separate male and female gametophytes.
- The male gametophyte produces pollen grains, which contain the male gametes (sperm).
- The female gametophyte develops in the ovule and contains the egg.
- Pollination occurs when pollen is transferred to the ovule.

2. Formation of the endosperm:
- After pollination, the pollen tube grows and delivers the sperm to the egg.
- Fertilization occurs when the sperm fuses with the egg, forming a zygote.
- In gymnosperms, the endosperm is formed as a result of double fertilization.
- The second sperm fuses with the central cell of the female gametophyte, resulting in the formation of the endosperm.

3. Chromosome number in the root:
- The root of a gymnosperm develops from the zygote, which is formed by the fusion of a sperm and an egg.
- Since the haploid number of chromosomes in gymnosperms is 12, the zygote will have a diploid number of chromosomes, which is double the haploid number.
- Therefore, the root will have 24 chromosomes.

4. Chromosome number in the endosperm:
- The endosperm is formed by the fusion of a sperm and the central cell of the female gametophyte.
- The central cell is also haploid, so the sperm will double the chromosome number.
- Therefore, the endosperm will also have 24 chromosomes.

In conclusion, the number of chromosomes in the root and endosperm of a gymnosperm is 24 and 24, respectively. Thus, the correct answer is option C) 24, 12.