The correct option is D. I am seeking employment that will allow me to explore my horticulture interests.
Changing the phrases "to get", "digging around" and "planting things" to more formal options like "seeking employment", "allow me to explore my horticulture interests" makes the sentence more formal.
Moreover, the other options contain informal or vague phrases such as "mess around" (Option A), "like working outside" (Option B), "let me plant things" (Option C).