Size of short integer and long integer would vary from one platform to another.

True, When a function is declared inside the source file, that function(local function) get a priority than the extern function. So there is no need to declare a function as extern inside the same source file.

6.68 is double.
6.68L is long double constant.
6.68f is float constant.
6.68LF is not allowed in c.

int length; denotes that variable length is int(integer) data type.

char int; here int is a keyword cannot be used a variable name.

int long; here long is a keyword cannot be used a variable name.

float double; here double is a keyword cannot be used a variable name.

So, the answer is int length;(Option A).

In 2 and 3 semicolon are missing in structure element.

float a = 3.14; a = a%3;

float a = 3.14; a = a%3; gives "Illegal use of floating point" error.

The modulus (%) operator can only be used on integer types. We have to use fmod() function in math.h for float values.

False, The range of float is -3.4e+38 to 3.4e+38.

True, we can use long double; if double range is not enough.

double = 8 bytes.
long double = 10 bytes.

False, The range of double is -1.7e+308 to 1.7e+308.

True,
float = 4 bytes.
double = 8 bytes.

True, we can find the size of short integer and long integer using the sizeof() operator.
Example:

#include<stdio.h>
int main()
{
    short int i = 10;
    long int j = 10;
    printf("short int is %d bytes.,\nlong int is %d bytes.",
            sizeof(i),sizeof(j));
    return 0;
}

Output:
short int is 2 bytes.
long int is 4 bytes.