Range of float id -2.25e+308 to 2.25e+308

True, Depending on the operating system/compiler/system architecture you are working on, the range of data types can vary.

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.

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.

Step 1: int i=4, j=-1, k=0, w, x, y, z; here variable i, j, k, w, x, y, z are declared as an integer type and the variable i, j, k are initialized to 4, -1, 0 respectively.

Step 2: w = i || j || k; becomes w = 4 || -1 || 0;. Hence it returns TRUE. So, w=1

Step 3: x = i && j && k; becomes x = 4 && -1 && 0; Hence it returns FALSE. So, x=0

Step 4: y = i || j &&k; becomes y = 4 || -1 && 0; Hence it returns TRUE. So, y=1

Step 5: z = i && j || k; becomes z = 4 && -1 || 0; Hence it returns TRUE. So, z=1.

Step 6: printf("%d, %d, %d, %d\n", w, x, y, z); Hence the output is "1, 0, 1, 1".