What is a deadlock in the context of database transactions?

Question $4$ options:

a$)$

A situation where a transaction cannot proceed because of unavailability of resources

b$)$

A situation where two or more transactions are waiting for each other to release a lock

c$)$

A situation where a transaction must be rolled back due to a system failure

d$)$

A situation where a transaction must wait for a time$-$out period to elapse