'Ripen' is a verb. 'Ripe' is an adjective. Use: 'Ripe'.
I agree with Ahmed_btm. Interestingly, "ripen" can be changed from a verb into an adjective (a participial adjective) by adding -ed: "ripened." Thus, we can speak of there being many ripe guavas on the guava tree, or of there being many ripened guavas on the guava tree. "Ripened" is commonly found in hyphenated constructions, like "sun-ripened tomatoes" and "tree-ripened guavas." In your sentence, bear_bear, "ripe" would be the usual choice, even though both words work. If the guavas on the guava tree were not yet fully ripe, you could speak of their being many ripening guavas on the guava tree (another participial construction).