Either (a) or (c) is acceptable. From Onestopenglish.com: "The difference between will and going to (not to mention other ways of expressing futurity) is so subtle that it cannot be easily conveyed through rules or isolated examples. Moreover, often the two forms are interchangeable (I think it will rain/I think it’s going to rain) or the meaning overlaps to such an extent that there is no risk of the learner being misunderstood."
Visit the site (and others) for additional explanation of the rules for use of either form.
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