Critically explain the following econometric models:
a. Vector Autoregression Model (VAR)
b. Generalized Autoregression Conditional Heteroskedasticity Model (GARCH)
c. Exponential Generalized Autoregression Conditional Heteroskedasticity Model (EGARCH)
d. Generalized Autoregression Conditional Heteroskedasticity in Mean Model (GARCH-M)