When a broad cast packet is generated in a network it is forwarded to all the prots of a switch or hub because broad cast packets have no IP addres. Therefore switch or hub run under one broadcast domain.
When two hosts access the network at the same time (Suppose the entire network is free and PC1 and PC5 are accessing the ntwork at the same time) a collision occurs. In case of hub it run under one collision domain because it is a physical star topolgy device but working as bus but in case of switch, it tries prot to prot link there fore each port of a switch is a collision domain i.e. no of ports = no. of collision domain.
Routers don't support broadcast and broadcast packets are dropped by router.