1. Write a recursive function in C that takes an integer as an argument and returns the factorial of that number.

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int factorial(int n) {    if (n == 0) {        return 1;    }    return n * factorial(n - 1); }

1. Write a recursive function in C that takes a positive integer as an argument and returns the nth Fibonacci number.

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int fibonacci(int n) {    if (n <= 1) {        return n;    }    return fibonacci(n - 1) + fibonacci(n - 2); }

1. Write a recursive function in C that takes a string as an argument and returns the number of times a given character appears in the string.

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int countChar(char* str, char c) {    if (*str == '\0') {        return 0;    }    if (*str == c) {        return 1 + countChar(str + 1, c);    } else {        return countChar(str + 1, c);    } }

1. Write a recursive function in C that takes an array of integers and its size as arguments, and returns the largest element in the array.

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int findMax(int* arr, int size) {    if (size == 1) {        return arr[0];    }    int max = findMax(arr, size - 1);    if (arr[size - 1] > max) {        max = arr[size - 1];    }    return max; }

1. Write a recursive function in C that takes a number as an argument and returns true if the number is a prime number, false otherwise.

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bool isPrime(int n, int i) {    if (n <= 1) {        return false;    }    if (i == 1) {        return true;    }    if (n % i == 0) {        return false;    }    return isPrime(n, i - 1); } bool isPrimeNumber(int n) {    return isPrime(n, n / 2); }

Please note that these are just sample solutions to the problems, and there are many ways to solve a problem using recursion in C. Also, these problems are not meant to be difficult, the goal is to give you an idea of how recursion works in C and how it can be used to solve problems.