Your parnter opens 2C multi. This is either 6-card ♦ and weak (5 to 10pts. Does not really need good ♦'s) or anything strong.
? : You can bid 3H forcing to show good 5-card H and intersted in game against a good weak 2♦
Or you can bid 2NT. Your parnter will then bid 3D with really weak hand and 3C when he is a bit stronger (8-10) when he gets the chanche
?? : 5D to stop or 4NT RKC (and hope that partner does not have S Ace)?
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Our bidding was:
2C*-(2S)-2NT-(3S)
3D-(4S)-4NT-(DBL)
5D-(DBL) all pass
(parnter forgot about ROPI)
...but the following looks better?:
2C*-(2S)-3H-(3S)
3D-(4S)-5D-(DBL) all pass
OR
2C*-(2S)-3H-(3S)
4H-(4S)-5H-(DBL) all pass

Help

2C*-(2S)-?-(3S)
3D-(4S)-??