In the round of 2's the High Jokers are the top trump, followed by the low trump, followed by the 2's of trump, followed by all other 2's, then the Ace to 3 of trump. In the round of 2, the trump number is 2. To solidify trump, a second 2 of the same suit may be played by your partner or yourself. It is the round of 2. After 5 cards are dealt, player A lays a 2 of Hearts.
Hearts is now trump.
However, after 9 cards are dealt, Player C plays both 2's of Clubs. Clubs are now trump and cannot be changed. Player A could have solidified trump by playing the other 2 of Hearts before the pair of Clubs were laid. If Player A,B,C or D had picked up a 2 of Diamonds as their first card and immediately laid it down as trump, it could not have been changed by anyone. Lay down the round of trump on the first card - trump cannot be changed 2.