The transaction command is used to group events that share a common value for one or more fields into transactions2. The transaction command assigns a transaction ID to each group of events and creates new fields such as duration, eventcount and eventlist for each transaction2. To identify all of the contributing events within a transaction that contains at least one REJECT event, you can use the following syntax: index=main | transaction sessionid | search REJECT2. This search will first group the events by sessionid, then filter out the transactions that do not contain REJECT in any of their events2. Therefore, option B is correct, while options A, C and D are incorrect because they do not follow the correct syntax for using the transaction command or the search command.