What will be the best case time complexity of the following code?

#include<bits/stdc++.h> 
using namespace std; 
void func(char* str2, char* str1) 
{ 
	int m = strlen(str2); 
	int n = strlen(str1); 
 
	for (int i = 0; i <= n - m; i++) 
        { 
		int j; 
 
 
		for (j = 0; j < m; j++) 
			if (str1[i + j] != str2[j]) 
				break; 
 
		if (j == m) 
			cout << i << endl; 
	} 
} 
 
int main() 
{ 
	char str1[] = "1253234"; 
	char str2[] = "323"; 
	func(str2, str1); 
	return 0; 
}

A. O(n)

B. O(m)

C. O(m * n)

D. O(m + n)

Answer: Option B