Ensemble forecasting consists in :

$($c$)$ Running the same numerical weather prediction model a number of times to verify that it yields exactly the same results.

$($e$)$ Running several numerical weather prediction models for different variables and combining the results into a single output.

$($b$)$ Running a numerical weather prediction model a number of times with slight variations in initial conditions to compare the different outputs.

$($d$)$ Running several numerical weather prediction models over different geographical areas and combining the results into a single global output.

$($a$)$ Running a number of different computer models at exactly the same time to compare their speed.