Double.
This is an important case that many club players get wrong. The temptation is to bid 2♥, but double is a far superior bid. Firstly, the double implies 4 hearts.
Second, and even more important, partner may well have good diamonds and want to extract a penalty. They cannot double themselves because that would be for takeout (and show a heart suit). They are desperate for us to double so they can pass!
This is another case where a double ostensibly for takeout, may well get converted to penalties by a pass from partner.
Note also, that since a double implies 4 hearts, bidding hearts would show a 5+ suit.
