Synonyms are those which can compensate a word for another because they are same in meaning. Graphic refers to commodity that's depicted through plates or written formats; which is easily written to grasp fluently. This is related to a pictorial representation of certain motifs so that we can work on our photographic memory which has a long term effect. 
Complete answer 
Now, from the options given above, 
Option a, Obscure, is easily the contrary of the word visual, depicting a vague picture or circumstance, thereby, banning itself from the option of being an answer. Eg The plan for the house is obscure. 
Option b, Implicit, means those which are obsolete or inferred upon and hence isn't the answer. Eg His words are implicit in regard to his situation. 
Option d, vague, as mentioned over is those pertaining to unclear or indefinite in nature. It can also describe a kind of indecisiveness or a lack of proper perception. Eg the idea of a sand marriage is vague in the eyes of my mama . 
Option c, visual, is the correct answer since visual refers to those which are pictorial in nature furnishing a clear view on what the idea is; Other words that can relate to the same are elucidative, diagrammatic, representative, graphic , and indeed suggestive.