Which of the following is the symbol used to redirect standard input? (programming in C)?

Its <

For example if you have a program, you can redirect standard input by doing:

program.exe < inputfile.txt

and the program will read input from inputfile.txt rather than from a keyboard (aka 'standard input')

It's <

myprogram.exe < afile.txt

Rather than taking it's input from the keyboard,

myprogram.exe will get the content of the afile.txt

sent as input.

This is part of the operating-system, not C itself.

(In Unix/Linux & puts the program in the background

for execution, so the shell can be used for other programs,)

Neither. You can re-direct stdin using freopen.

EDIT: If you're too stupid to realise that whatever shell you use has nothing to do with C, then you're either confused or mentally retarded. In C, & is the bit-wise and operator, and < is the relational less-than operator. Neither are used for redirecting stdin. If you want to redirect stdin within a C program, you can do so with freopen, in a similar manner as shown below:

#include <stdio.h>

/* ... */

if(!freopen("file", "r", stdin))

   /* Handle Error */

This will achieve a similar result as executing a program from a shell, like so: "./program < file", except that it's done within C itself, thus being relevant to your poorly worded question.

I recommend you get a book on either UNIX or C. Whatever you think you're trying to learn.