What is the difference between balanced and unbalanced panels?

Select one:

a. Unbalanced panels have some observations missing; balanced panels do not.

b. Balanced panels are demographically representative of the population being studied; unbalanced panels are not.

c. Balanced panels have an equal number of observations above and below the mean of the dependent variable; unbalanced panels are skewed.

d. A balanced panel has T = N; an unbalanced panel has N>T or N