With phrases such as 'es aconsejable que' (= it is recommended that), you need to use the subjunctive form of the verb in Spanish. Desayuno, desayunas, and desayunan are all indicative present forms of the verb desayunar, but desayunemos is not - it is subjunctive (the normal, indicative form would be desayunamos).
Thus, the correct answer is desayunemos.
