Partner/Declarer opens 1 club
RH opponent overcalls 2 diamonds
Responder has a balanced 9 HCPs:
S: K J x x
H: x x x
D: Q J x
C: Q x x
Should responder bid or pass? If bid, what is appropriate?
Given these assumptions:
Then my recommendation is to make a Negative Double.
My expectation is that when both majors are unbid, as here, then such a double at this level promises 4-3 or better in the majors. Some partnerships require 4-4 but that seems excessive and over-cautious for today's competitive auctions.
Additionally, you are better than a minimum 1-over-1 response would have shown, with support for partner's clubs if that suit is real. You also have the possibility of suggesting notrump as a denomination, but have no interest in attempting to defend 2D doubled.
If for some reason you are not playing Negative Doubles as a partnership agreement, then my advice is to start doing so. As this hand illustrates, you are severely handicapped in competitive auctions without this convention in your arsenal.
Meanwhile, 2NT is probably the best call you can make: showing (in a competitive auction) 10-11 HCP balanced and one stopper in the overcalled suit. It hides the nice 4-card Spade suit, but that is the price for playing 5-card Majors without Negative Doubles.
Note that in general for competitive auctions, as here, many strong bids become invitational instead - the reason is that a cue bid of the opponents suit is available to show all strong hands lacking interest in a penalty double. This helps to alleviate the bidding room stolen by the opponents overcall.
If you opponent had bid ONE diamond, I would bid one spade. I would do this even with a weaker hand than yours (take away a queen).
But since your opponent bid TWO diamonds, the "standard" response is a negative double, as recommended by Pieter.
This bid has two meanings. The first is, I'd rather not play in "your" suit (clubs) nor the opponent's suit (diamonds). That leaves spades and hearts.
The second meaning applies in your case: The opponents bid over my suit. So "go back" and think in terms of my bidding one spade (or one heart). I'm not "legally" allowed to say this, but I am allowed to say "double," which in this context, has the same meaning.
The negative double would be more emphatic if partner had opened one HEART, and opponent had bid two diamonds. Here, the clear implication is that I have four of the remaining major, don't want to bid two spades, and am bidding "one spade" in a roundabout way.