C++ solution - use vector to express two dimensional array, and then, define two vectors for neighbors - 8 neighbors, two queues.

ConnectedCellInaGrid4.cpp

#include <cmath>
#include <cstdio>
#include <vector>
#include <queue>
#include <iostream>
#include <algorithm>
using namespace std;


int main() {
    int M, N;
    cin >> M >> N;
    vector<int> grow(N+2,0);
    vector<vector<int> > grid(M+2, grow);
    
    for(int y=1;y<=M;y++){
        for(int x=1;x<=N;x++){
            cin >> grid[y][x];
        }
    }
    
    vector<int> dx = {1, 1, 0, -1, -1, -1, 0, 1};
    vector<int> dy = {0, 1, 1, 1, 0, -1, -1, -1};
    
    int best_comp = 0;
    for(int y=1;y<=M;y++){
        for(int x=1;x<=N;x++){
            if(grid[y][x]==1){
                int cur_comp = 0;
                queue<int> bfsx;
                queue<int> bfsy;
                bfsx.push(x);
                bfsy.push(y);
                grid[y][x] = 0;
                while(!bfsx.empty()){
                    cur_comp++;
                    int curx = bfsx.front();
                    int cury = bfsy.front();
                    bfsx.pop(); bfsy.pop();
                    
                    for(int i=0;i<8;i++){
                        int nextx = curx+dx[i];
                        int nexty = cury+dy[i];
                        if(grid[nexty][nextx]==1){
                            bfsx.push(nextx);
                            bfsy.push(nexty);
                            grid[nexty][nextx]=0;
                        }
                    }
                    
                }
                
                if(cur_comp > best_comp) best_comp = cur_comp;
            }
        }
    }
    
    cout << best_comp << endl;
    
    return 0;
}