Answer:
It is possible that the onion cell with 8 chromosomes could have been produced by mitosis. During mitosis, the chromosomes duplicate and the resulting daughter cells have the same number of chromosomes as the parent cell.
However, it is also possible that the onion cell with 8 chromosomes could have been produced by meiosis. Meiosis involves two rounds of cell division, and the number of chromosomes is reduced by half during the first division. If the onion cell with 8 chromosomes is one of the daughter cells produced by meiosis, then it is possible that the parent cell had 16 chromosomes.
Therefore, without further information about the history of the cell, it is not possible to determine with certainty whether the onion cell with 8 chromosomes was produced by mitosis or meiosis.