The most likely explanation for the piglet having a straight tail is that a genetic mutation occurred in the gamete.
Inherited traits, such as tail shape, are typically determined by genes passed down from the parents to their offspring. If both parent pigs have curly tails, it suggests that curly tail is the dominant trait in their genetic makeup. However, occasionally, genetic mutations can occur spontaneously, resulting in a change in the genetic information. These mutations can alter the expression of traits, leading to variations in the offspring.
In this case, it is likely that a genetic mutation occurred in the gamete (sperm or egg cell) of either the mother or the father pig, resulting in the piglet having a straight tail instead of the curly tail inherited from its parents. This mutation could have affected the genes responsible for tail shape, causing a deviation from the expected phenotype.
The other options mentioned—having a different father, the mother's diet during pregnancy, or multiple divisions in mitosis—do not typically explain the occurrence of a trait that deviates from the parents' traits in the same generation. Genetic mutations are a more plausible explanation for such variations in inherited traits.