Answer:
#include<iostream>
using namespace std;
int CountCharacters(char userChar, const string inputstr){
    int k = 0;
    int iter = 0;
    for (iter = 0; iter < inputstr.size(); iter++){
        if (inputstr[iter] ==  userChar){
            ++k;        }}
    return k;}
int main(){
    string str;
    char userChar[1];
    cout<<"Char: ";    cin>>userChar;
    cin.ignore();
    cout<<"String: ";   getline(cin, str);
    cout<<CountCharacters(userChar[0],str);
    return 0;}
Explanation:
Written in C++:
The function is defined here:
int CountCharacters(char userChar, const string inputstr){
This initializes a count variable k to 0
    int k = 0;
This initializes an iterating variable iter to 0
    int iter = 0;
This iterates through the characters of the string being passed to the function
    for (iter = 0; iter < inputstr.size(); iter++){
This checks for matching characters
        if (inputstr[iter] ==  userChar){
If found,  k is incremented by 1
            ++k;        }}
This returns the total count
    return k;}
The main begins here
int main(){
This declares a string variable
    string str;
This declares a character variable
    char userChar[1];
This gets input for the character variable
    cout<<"Char: ";    cin>>userChar;
This lets the user get another input
    cin.ignore();
This gets input for the string variable
    cout<<"String: ";   getline(cin, str);
This calls the function and return the count of character in the string
    cout<<CountCharacters(userChar[0],str);