/*分配一段区域，存放一个学生信息。*/
#include <stdio.h>
main()
{
      struct data
      {
             int month;
             int day;
             int year;
      };
      struct stu{
             char *name;
             char *sex;
             struct data *birthday;
             float score;
             } *ps;
            
             ps = (struct stu*)malloc(sizeof(struct stu));
             ps->name = "MrDong";
             ps->sex = "MEN";
             ps->birthday = 06,11,1995;
             ps->score = 100;
              
             printf("name = %s\nsex = %s\n",ps->name,ps->sex);
             printf("birthday = %d\nscore = %f\n",ps->birthday,ps->score);
             system("pause");
            
}
/* 输出结果为：
name = MrDong
sex = MEN
birthday = 6
score = 100.000000
请按任意键继续. . .

*/

压根就没有传进来，struct data *birthday;是个结构体指针，
ps = (struct stu*)malloc(sizeof(struct stu)); 分配了空间没错，
但分配完后，此时ps->birthday指向哪里？你并没有分配struct data空间给它，变成了野指针。。

 ps->birthday = 06,11,1995;结构体成员变量并不能这样赋值。你的编译器只好把第一个数扔给了指针birthday, 但多半会警告你这个地方有问题。VC里直接指出该行编译错误
error C2440: '=' : cannot convert from 'const int' to 'struct main::data *'

#include <stdio.h>
main()
{
      struct data
      {
             int month;
             int day;
             int year;
      } *bir;
      struct stu{
             char *name;
             char *sex;
             //struct data *birthday;
            
             float score;
             } *ps;
             bir = (struct data*)malloc(sizeof(struct data));
             ps = (struct stu*)malloc(sizeof(struct stu));
             ps->name = "MrDong";
             ps->sex = "MEN";
             //ps->birthday = 06;
             bir->month = 06;
             bir->day = 19;
             bir->year = 1995;
             ps->score = 100;
              
             printf("name = %s\nsex = %s\n",ps->name,ps->sex);
             printf("birthday:%d-%d-%d\n",bir->month,bir->day,bir->year);
             printf("score = %f\n",ps->score);
             system("pause");
            
}

结构体可以嵌套使用，你可以不改动原结构体的结构。分配了ps后，加一句，
ps->birthday=(struct data*)malloc(sizeof(struct data));

后面引用时你可以写成这样:
ps->birthday->month = xxx;
j加油

根据你的提示，做到这种 效果，感觉还不是很美，能不能一次实现  ps->birthday 可以把年月日等信息都传进去呢。。

你可以修改stu结构如下：
      struct stu{
             char *name;
             char *sex;
             struct data br;
             float score;
         void (*birthday)(int, int, int, struct stu *);
             } *ps;
并把二个结构放到main函数外面。然后在结构下面，加入
void imp_birthday(int month, int day, int year, struct stu *ps)
{
    ps->br.month = month;
    ps->br.day   = day;
    ps->br.year  = year;
}

main函数里这样修改
void main()
{
    ps = (struct stu*)malloc(sizeof(struct stu));
    ps->birthday = imp_birthday;
    ps->name = "MrDong";
    ps->sex = "MEN";
    ps->score = 100;
    ps->birthday(06, 11, 1995, ps); //这是你想要的结果么？
              
    printf("name = %s\nsex = %s\n",ps->name,ps->sex);
    printf("birthday = %d\nscore = %f\n",ps->br.day,ps->score);
    system("pause");
}

说实话，你的程序我有些疑问（很大程度上没看懂哎）
1、void imp_birthday(int month, int day, int year, struct stu *ps)
{
    ps->br.month = month;
    ps->br.day   = day;
    ps->br.year  = year;
}

四个参数传了三个，最后一个怎么理解呢？
2、

你想达到什么目的？看你的档案，你是学过C++的，返回来问这种问题？

确实学过C++，那时候是大一学的，现在都大三了，为了给大四做准备，我准备把C语言弄透，弄细。

你这程序在数据结构上就是自己制造麻烦。