Why am I getting segmentation fault in search_lastname function?

The following program accepts data from the user of the student's first name, last name and grades. The search_lastname function is expected to search records for the specified last name. But it gives a segmentation fault. Another feature is to print the entries, and the cases for the switch were created for additional features not yet added to the program.

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
char **first_name, **last_name;
float *score;
int entry;
void print();
void search_lastname(char search_last_name[21]);
int main()
{
int i,option;
char search_last_name[21];
do
    {
    printf("\nPlease indicate the number of records you want to enter (The minimum number of entries is 5) :");                     
    scanf("%d",&entry);
    }
while(entry<=4);
score=(float*)malloc(entry*sizeof(float));
first_name=(char**)malloc(entry*sizeof(char*));
last_name=(char**)malloc(entry*sizeof(char*));
for(i=0;i<entry;i++)
    {
    first_name[i]=(char*)malloc(21*sizeof(char));
    last_name[i]=(char*)malloc(21*sizeof(char));
    }
printf("Please input records of students (enter a new line after each record), with following format first name last name score \n");
for(i=0;i<entry;i++)
    {
    scanf("%s%s%f",&first_name[i],&last_name[i],&score[i]);                                                             /*Input of records itself*/
    }
do
{
    printf("\nPlease choose the appropriate options :");                                                /*Switch statement for choosing options.*/
    printf("\nPrint records (press 1)\nAdd a new record (press 2)\nDelete records (press 3)\nSearch by last name (press 4)\nSort by score (press 5)\nSort by last name (press 6)\nFind Median score (press 7)\nExit the Program (press 0)");
    scanf("%d",&option);
    switch(option)
        {
        case 1  :print();
            break;
        case 2  :
            break;
        case 3  :
            break;
        case 4  :
            printf("\n Please enter the last name you want to search: ");
            scanf("%s",search_last_name);
            search_lastname(search_last_name);
            break;
        case 5  :
            break;
        case 6  :
            break;
        case 7  :
            break;
        case 0  :
            break;
        default : 
            printf("\n Please enter a valid option.");
        }
}
while(option>=1 && option<=7);
return 0;
}

void print()
{
int i;
printf("\n The records of students are as follows :");
for(i=0;i<entry;i++)
    {
    printf("\nFirst name:%s, Last name:%s, Score:%f\n",&first_name[i],&last_name[i],score[i]);
    }
}


void search_lastname(char search_last_name[21])                                     /*Funtion to search by last name*/  
{
int i,counter=0;
for(i=0;i<entry;i++)
    {
    if(strcmp(search_last_name,last_name[i])==0)
        {
        printf("\nFirst name:%s, Last name:%s, Score:%f\n",first_name[i],last_name[i],score[i]);
        }
    }

}