Array indexing in C starts from 0. str[0]='H', str[1]='e', str[2]='l', str[3]='l', str[4]='o', str[5]='\0'. Therefore, str[1] is 'e'.

strcmp() is the standard library function used to compare two strings. It returns 0 if strings are equal, negative if first string is less, and positive if first string is greater. It's declared in string.h.

p is a pointer to x, so &x gives the address of x. *p (dereferencing) gives the value stored at that address, which is 5. The output will be 5.

In C, identifiers can only contain alphanumeric characters (a-z, A-Z, 0-9) and underscores (_), and must start with a letter or underscore. The dollar sign (\() is not allowed, making 'my\)var' invalid.

x++ is post-increment (returns 5, then x becomes 6), ++x is pre-increment (x becomes 7, then returns 7). The order of evaluation in printf is implementation-dependent, but typically right to left, giving 5 7.

Structure allocates separate memory for each member (total size = sum of all members). Union shares a single memory location among all members (total size = size of largest member). This is a fundamental difference in memory allocation.

The correct syntax is 'void function(void);' where 'void' in parentheses explicitly states no parameters. Option B is also valid in C (defaults to no parameters), but option A is more explicit and portable across C standards.

typedef creates an alias (synonym) for existing data types. For example, 'typedef int Integer;' creates 'Integer' as an alias for 'int'. This improves code readability and portability. It doesn't create entirely new types, but provides alternative names.

In C, 'string' is not a primitive data type. C uses 'char' arrays to represent strings. The valid primitive data types are int, float, double, char, void, and their variants.

Variable 'a' is initialized to 10, 'b' is initialized to 20. When c = a + b, c becomes 10 + 20 = 30. Therefore, printf outputs 30.