Do you mean which fouls result in an untimed down? In that case, any fouls, except those which result in a safety, unsportsmanlike conduct, non-player fouls, live ball fouls treated as dead ball fouls, and succeeding spot fouls. Fouls resulting in a loss of down do not extend the period, unless the offended team chooses to extend the period. In any case, NCAA changed the untimed down rule this year to only apply to fouls at the end of a half.
In any period, if a team scores a touchdown on the last play, the period will be extended for the try.
