Answer
(a) Organic chemistry is the study of compounds which contain mainly $C-C$ and $C-H$ bonds. Some organic compounds contain bonds with some other atoms, mainly: $oxygen$,$nitrogen$,$sulfur$,$phosphorus$ and $halogens$.
(b) While some theories about life based around atoms other than carbon exist life as we know it would not exist without organic chemistry.
(c) $The$ $Vital$ $Force$ $Theory$ was debunked in 1828 when Friedrich Wöhler was able to synthesize urea, the main compond of urine, starting from inorganic compounds.
Work Step by Step
I mostly looked up the Oxford dictionary. For answers (b) and (c) I also needed to do some research on an encyclopedia.
