why this shell command wrong

The correct line would be:

find -name "*.c" -exec ls -l \{\} \;

or

find -name "*.c" -exec ls -l '{}' ';'

or

find -name "*.c" -exec ls -l '{}' \;

...
You see the pattern?

Reasoning?

1. the shell splits the command line into chunks (1st chunk is the command, the rest are the individual arguments to the command)
2. since the shell is a language in it self, it has some relevant punctuation and operator symbols - among others, { } and ; have a meaning to the shell
3. since the find command also needs for some operations the {} and ; as own arguments, one must make sure that the {} and the ; are not interpreted by the shell, but are rather passed to the find command. There are two possibilities to achieve that: escaping or placing into a literal string.
4. escaping is placing a \ before one character - the character then is not interpreted by the shell and is passed as-is (without leading \)
5. '...' strings are passed as-is, after stripping off the leading and trailing single quote
6. "..." strings get evaluated by the shell for variable substitution and other functions and only then are taken (by stripping off the quotes)

Note: If you want a text be passed unchanged to a command, use single quoted string ('...'). If you would place the {} or the ; into "{}" ";", they are interpreted by the shell and thus might not be passed unchanged to the find command.