问题在于，您在每个if/else if分支中声明了一个新的、不同的（如果名称相似）char *description，因此，顶层description实际上保持未初始化状态。
因为你要返回一个常量字符串，你可以

const char *get_growth_rate_description(double growth_rate) {
  if (growth_rate < 0) {
    return "Negative";
  }
  if (growth_rate < 1) {
    return "reasonable";
  }
  if (growth_rate < 2) {
    return "ambitious";
  } 
  if (growth_rate < 4) {
    return "high";
  }
  return "unreasonable";
}

char *description;未初始化。
然后在if语句作用域中定义新的description变量，但是它们在这些作用域之外停止存在，并且返回在函数作用域中定义的description，而这个变量没有初始化。
您需要从分配中删除char *

const char *get_growth_rate_description(double growth_rate) {

  char *description;
  double growth_percentage = growth_rate * 100;

  if (growth_rate < 0) {
    description = "Negative";
  } else if (growth_rate >= 0 && growth_rate < 1) {
    description = "reasonable";
  } else if (growth_rate >= 1 && growth_rate < 2) {
    description = "ambitious";
  } else if (growth_rate >= 2 && growth_rate < 4) {
    description = "high";
  } else {
    description = "unreasonable";
  }
  return description;
}

对于初学者，变量growth_percentage声明如下

double growth_percentage = growth_rate * 100;

未在函数中使用。
由于函数的返回类型是const char *，因此将变量description也声明为具有类型const char *将在逻辑上更加一致。

const char *description;

在每个if语句的子语句中，你用一个名字description来声明新变量，这个新变量隐藏了在if语句集之前声明的变量，所以这个变量保持未初始化状态，函数返回的就是这个变量。
与在of语句中声明新变量不同，您需要使用assignment语句将字符串字面值赋给在if语句集之前声明的变量。

const char * get_growth_rate_description(double growth_rate) {

  const char *description;
  double growth_percentage = growth_rate * 100;

  if (growth_rate < 0) {
    description = "Negative";
  } else if (growth_rate >= 0 && growth_rate < 1) {
    description = "reasonable";
  } else if (growth_rate >= 1 && growth_rate < 2) {
    description = "ambitious";
  } else if (growth_rate >= 2 && growth_rate < 4) {
    description = "high";
  } else {
    description = "unreasonable";
  }

  return description;
}

或者你可以把函数写成

const char * get_growth_rate_description(double growth_rate) {

  const char *description = "unreasonable";
  double growth_percentage = growth_rate * 100;

  if (growth_rate < 0) {
    description = "Negative";
  } else if (growth_rate >= 0 && growth_rate < 1) {
    description = "reasonable";
  } else if (growth_rate >= 1 && growth_rate < 2) {
    description = "ambitious";
  } else if (growth_rate >= 2 && growth_rate < 4) {
    description = "high";
  }

  return description;
}