We know the baseband line at one time send or receive data(half duplex). If more than one host send data at same time may occur collision. In bus topology see the same thing. Inspite this in home, college, university uses this lan technologies. But there doesn't occurred any collision and they did multiple task parallelly. My question is that what is the actual reason to not occurred collision after using LAN(ring topology) in (home, college, university)?
Baseband is a 'plain' signaling method (in contrast to using one or more carrier frequencies), entirely unrelated to the access method in a network.
What you're referring to is CSMA/CD used with early Ethernet. CSMA/CD is practically extinct with micro-segmented, fully-switched networks utilizing full duplex being the norm (due to switches having supplanted primitive repeater hubs, or even the early shared-wire technology used with coax). There are no collisions on full-duplex links.
Ethernet generally uses a star or tree (=multi-star) topology. Rings or chains should generally be avoided for performance and reliability reasons.