The correct answer is A. Relocating/subject
Explanation:
In grammar, a gerund is a word that is formed by adding "-ing" at the end of it and that mainly work as a noun instead of a verb, but that can also have other functions in sentences. Considering this, the only noun in "Relocating because of a parent's job change can be difficult for school-aged children" is "Relocating" as this is the only word that ends in -ing. Besides this, it is important to consider the function of it in the sentence, about this most sentences, are composed by a subject which is the agent that expresses or carries on the action, the main action expressed by a verb and a complement which includes people or objects affected by the action, places, time and other element. In the case of "Relocating" this gerund functions as a subject as it is the agent of the sentence, this means "Relocating" is the idea that is connected to the main verb "can be". Thus, the gerund in this sentence is "Relocating" and it works a the "subject".