Router C
Router C send routes send the following routes to router B
250.0.0.0/8
100.0.0.0/8

Router B (SRX)
Without the conditions applied the SRX would send the following routes to router A
250.0.0.0/8
100.0.0.0/8
After the following configurations applied the SRX would install a default route into its own routeing table based on the presence of 250/8 and will also advertise the a default and 100/8 to router A

Router A
Router A routes without conditions met
250.0.0.0/8 1.1.1.2
100.0.0.0/8 1.1.1.2
Router A routes with conditions met
0.0.0.0/0 1.1.1.2
100.0.0.0/8 1.1.1.2

Filter on router A to ensure it only accepts the routes define in route-policy RP-WAN-IN
bgp 1
 peer 1.1.1.2 as-number 2
 #
 address-family ipv4 unicast
  network 10.180.0.0 255.255.0.0
  peer 1.1.1.2 enable
  peer 1.1.1.2 route-policy RP-WAN-IN import
#
route-policy RP-WAN-IN permit node 10
 if-match ip address prefix-list PL-WAN-IN
#
 ip prefix-list PL-WAN-IN index 10 permit 0.0.0.0 0
 ip prefix-list PL-WAN-IN index 20 permit 100.0.0.0 8




#Peer Statement # Router B SRX

set protocols bgp local-as 2
set protocols bgp group AtoB type external
set protocols bgp group AtoB local-address 1.1.1.2
set protocols bgp group AtoB export CONDITIONAL_ROUTE #this advertises 0.0.0.0/8 1.1.1.2 and 100.0.0.0/8 1.1.1.2 to router A if they are present in the routing table
set protocols bgp group AtoB peer-as 1
set protocols bgp group AtoB neighbor 1.1.1.1

set policy-options policy-statement CONDITIONAL_ROUTE term 1 from route-filter 0.0.0.0/0 exact
set policy-options policy-statement CONDITIONAL_ROUTE term 1 from route-filter 100.0.0.0/8 exact
set policy-options policy-statement CONDITIONAL_ROUTE term 1 then accept
set policy-options policy-statement CONDITIONAL_ROUTE then reject


set routing-options generate route 0.0.0.0/0 policy GEN_DEFAULT # generates default towards the advertising router if conditions are met.

set policy-options policy-statement GEN_DEFAULT term 1 from protocol bgp
set policy-options policy-statement GEN_DEFAULT term 1 from route-filter 250.0.0.0/8 exact
set policy-options policy-statement GEN_DEFAULT term 1 then accept
set policy-options policy-statement GEN_DEFAULT then reject