#include <stdbool.h>
#include <stdio.h>

//gcf function - return gcd of two numbers
int gcd(int n, int m)
{
	int gcd, remainder;
	
	while (n != 0)
	{
		remainder = m % n;
		m = n;
		n = remainder;
	}
	
	gcd = m;
	
	return gcd;
}//end gcd function

int main (int argc, const char * argv[]) {
    // insert code here...
	//--declarations
	int number1, number2;
	int newNumber1, newNumber2;
    
	//--get user input
	printf("Enter a fraction: ");
	scanf("%d/%d", &number1, &number2);
	
	//--calculations
	//find the gcd of numerator and denominator
	//then divide both the numerator and denominator by the GCD
	newNumber1 = number1 / gcd(number1, number2);
	newNumber2 = number2 / gcd(number1, number2);
	
	//--results
	printf("In lowest terms: %d/%d", newNumber1, newNumber2);
}

To reduce a fraction to lowest terms, first we compute the GCD(greatest common divisor) of the numerator and denominator.
Then we divide both the numerator and denominator by the GCD.

Reduce a fraction to lowest terms

C