The correct answer is: to planning.
The verb “devote” means to give a lot of time and attention to something. In this sentence, the person is devoting their time to planning for the future. The preposition “to” is used to indicate the purpose of the action. In this case, the person is planning for the future.
The other options are incorrect because they do not make sense in the context of the sentence. The noun “plan” is not a verb, so it cannot be used to describe the action of devoting time to something. The infinitive “to plan” is also incorrect because it does not indicate the purpose of the action. The gerund “planning” is the correct answer because it indicates that the person is devoting their time to the action of planning.