Why is this wrong? What to do of the two variables in the if condition? How to fix this?

First of all, learn to indent properly your code, it show its structure and it helps reading and understanding. Do not try to pack things on same line, it don't make the code faster, just more difficult to read.

#include<stdio.h>
int main()
{
	int c=0,n,k,i=1;
	printf("Enter 'n'&'k'respectively:\n");
	scanf("%d %d",&n,&k);
	int t[n];
	printf("Enter %d numbers:\n",n);
	for(i=0;i<n;i++)
	{
		scanf("%d",t[i]);
	}
	printf("count of numbers divisible by %d is:\n",k);
	for(i=0;i<n;i++)
	{
		if(t[i]%k==0)  /*both are variables, what to do*/
			c=c++;
	}
	printf("%d",c);
}

Professional programmer's editors have this feature and others ones such as parenthesis matching and syntax highlighting.
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The correct syntax for

c=c++;

is

c++;

Advice: you don't need an array for this problem, each value you store in the array is reuse inly once.
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Quote:

but in the if condition both t[i] and k are constants, i dont know what to do!

The debugger will show you what is qrored in array t.
The debugger will help you to understand what is going on by showing you what it does.

There is a tool that allow you to see what your code is doing, its name is debugger. It is also a great learning tool because it show you reality and you can see which expectation match reality.
When you don't understand what your code is doing or why it does what it does, the answer is debugger.
Use the debugger to see what your code is doing. Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't find bugs, it just help you to. When the code don't do what is expected, you are close to a bug.