fmod(x,y) - Can be used to Calculates modulo of floating point divisions.

x modulo y = remainder of x / y.

C supports External, Internal and None types of linkage.

External Linkage consists of global, non-static variables and functions.
Internal Linkage consists of  static variables and functions with file scope.
None Linkage consists of Local variables.

_ (Underscore) is allowed in variable name.

extern keyword before any function in C denotes that, this function is global function and it is either defined externally to the current module, or in another file.

Here, both the functions are same but extern in first function declaration represents that this function is probably in another file. 

ceil() function is C gives rounded up value while floor() function gives rounded down value. 

So, ceil(3.77) would be 4.0

and, floor(3.77) would be 3.0

Hence, ceil(3.77) is correct answer.

See the program below to see the implementations of ceil() and floor() functions is C.

#include<stdio.h>
#include<math.h>

int main()
{
    printf("\n Ceil(3.77) = %f" , ceil(3.77));
 
    printf("\n Floor(3.77) = %f" , floor(3.77));    

    return 0;
}

Real number is superset of rational and irrational number. For irrational numbers, we are going to have infinite numbers after decimal. Since, there is no way to store infinite decimal numbers, we will choose the data type that allows us to store as much precision as possible.

Here long double is the highest possible data type to store decimal values. So we'll choose it.

long double takes 10 bytes of memory space.

In C Following is classification of data types. Here Primary data types are those which are not user defined and secondary are those which are user defined.

Primary data types

1. int
2. char
3. float
4. double
5. void

Secondary data types

1. Array
2. Pointer
3. Structure
4. Union
5. Enum

Based upon above classification we can deduce that option B is correct answer.  

extern int i is declaring a variable named i

extern is a keyword in C.

extern int m; is declaring a variable with name m.

double pow(double, double); is declaring a function.

float cube( float x ) { ... } is defining a function.

extern int x - Declared the variable.

int x = 24 - We initialized the value in it, hene we defined it.

In C / C++,  we need to mention complete prototype of the function during its declaration.