What is the best evidence telling you whether this cell is diploid or haploid?

As a result of mitosis and meiosis, haploid and diploid cells are formed.

The cell will be diploid if it will have two sets of chromosomes, that is the number of chromosomes will be double than that of haploid one. And all the chromosome pairs in diploid are considered homologous chromosomes. e.g. the somatic cells of the body are diploid cells.

Haploid cells will be those who do not have chromosomes in pairs, a single set of chromosomes is found in them. e.g. the haploid spores or gametes cells. The chromosome number in haploid cells are represented as 'n'. Haploid cells are produced via meiosis in cell division.

meiosis is the first to occur after the nuclei of opposite of opposite mating types fuse

because photosynthesis could not occur (apex)