The split in the Congress Party in 1969 primarily occurred due to the dominance of the Congress Syndicate, a group of senior party leaders who opposed the leadership of Indira Gandhi. Indira Gandhi’s attempt to centralize power and her push for greater control led to a clash within the party, eventually resulting in a split. This division led to the formation of two factions: the Congress (O) and Congress (I), with Indira Gandhi leading the latter.
Thus, the correct answer is (B) Dominance of the Congress Syndicate.