The first, and simplest, way to handle this is to set up a distribute list specifically blocking the routes that you don't want to redistribute. For example, on Router D, you could build the following distribute list:
access-list 10 deny 172.16.20.0 0.0.0.255 access-list 10 permit any !
router rip redistribute eigrp 100 distribute-list 10 out serial 0
Assuming that serial 0 is the link between Router D and Router E, this will resolve the problem. RIP will not advertise the 172.16.20.0/24 route from Router D to Router E. If you have more than one connection back into the RIP side of the network, it can be difficult to manage the distribution lists that must be maintained.
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