Answer

②Concealed

Commentary

In the last article, I said that if you wait  Ryanmen, you should  call for following Riichi basically.

How about this case ??

Using the statistical possibliity from the last article, let's calculate the rough expected value.

Right side player has 5 dora. Hanneman is confirmed　(Riichi ＋Dora 5).

Following Riichi in the 14th round, the deal in ratio is approximately 15%.

Expected value of Deal in is roughly 13,000 (Haneman + Riichi fee) x 15%＝1950.

Win ratio is roughly 45% x 2700 points (average score for Riichi only in case of hided dora x 3) is 1215.

The rest is the probability of your opponent's self-draw x 3000 or 4000, but this can vary greatly depending on how your opponent waits.

If the opponent wait for Ryanmen , the self drawing probability is 17%, so the probability is approximately 4000 (30% chance of having a concealed dora x 3, which means there is at least one concealed dora) x 17%,= 680.

In addition, this hand has a large number of safty tiles for the Riichi, so we can decide that fold is better for this hand.

However, 

① If you really don't want to be won by the right side player.

② I don't have a safety tile.

If you have such conditions, it is acceptable to go to Riichi even in such an extreme situation.

Even if you can see Dora 5, the difference in numbers is about this much, so you can see how strong Ryanmen following Riichi is.