The correct answer is (a).
The second central moment is the variance, which measures how spread out the data is around the mean. The third central moment is the skewness, which measures how asymmetrical the data is. If the second and third central moments are equal, then the data is symmetric.
A symmetric distribution is one where the mean, median, and mode are all equal. The data is evenly distributed on either side of the mean.
An asymmetric distribution is one where the mean, median, and mode are not all equal. The data is not evenly distributed on either side of the mean.
A positively skewed distribution is one where the tail on the right side of the distribution is longer than the tail on the left side. This means that there are more data points with values that are greater than the mean than there are data points with values that are less than the mean.
A negatively skewed distribution is one where the tail on the left side of the distribution is longer than the tail on the right side. This means that there are more data points with values that are less than the mean than there are data points with values that are greater than the mean.