Which configuration completes this task?

A network administrator has been asked to configure the most specific aggregate route to
summarize the four specific prefixes shown below:
10. .5.0/24
10. .12.0/24
10. .17.0/24
10. .21.0/24
Which configuration completes this task?

A.
[edit routing-options]
user@router# show
aggregate {
route 10. .0.0/19;
}

B.
[edit routing-options]

user@router# show
aggregate {
route 10. .0.0/20;
}

C.
[edit routing-options]
user@router# show
aggregate {
route 10. .0.0/21;
}

D.
[edit routing-options]
user@router# show
aggregate {
route 10. .0.0/18;
}

Explanation: