Answer (a) answers the question directly and is grammatically correct.
Answer (b) is more informal. It passes in conversation but I would not recommend that wording in a more formal document, unless it was a quotation. Max's answer explains why.
Answer (c) is also grammatically correct; it merely adds some information that wasn't initially requested.
Answer (d) is incorrect, because the verb is wrong. We do not say, "I having" anything. We might say "I am having" something, (such as, "I am having a party tomorrow," or "I am having a fit right now"), but I can think of no context where "I having" would be grammatically correct.
Incidentally, have is a very tricky word, as you might have noticed. It can mean own, or have in one's possession (as in, "I have a pencil"), but the word can be used in a host of other ways, too:
- I have an appointment
- I have a toothache
- I have an idea
- I have a new girlfriend
- I have a new roof on my house
- I have all my luggage packed
- I have my hair cut at the salon
- I have something I need to do tomorrow
- I have coffee in the morning
- I have a knack for finding good deals
- I have a lot of patience when it comes to answering questions
(All of those have shifted away from the notion of ownership or possession to some degree or another.)