ashu,
a. number is the only legal identifier in your list...
b. 12color starts with a number (which Java begins parsing as a numeric literal), so it is not a legal identifier.
c. abstract is a reserved
word (as in public abstract class someClass), so it is not a legal identifier either.
Legal identifiers can consist of upper and lower case characters, numerals, the dollar sign($) and underline(_), and can start with an upper or lower case character, a dollar sign($) or an underline(_), as long as the name of the identifier is not the same as a reserved word.
HTH,
-Nate