The correct option is D conserve
Dissipate means (with reference to a feeling or emotion) disappear or cause to disappear. The opposite of dissipate is conserve, or protect (something, especially something of environmental or cultural importance) from harm or destruction. Hence, Option D is correct. Among the rest of the options, utilise means make practical and effective use of, organise means arrange systematically, mobilise means to prepare and organize (troops) for active service. These options do not express this antonym, hence incorrect.