I tried to write a program for school grade and its reflecting A+ only

if and else if are not functions. They're language constructs.

You said you learned about if, but not enough. DO NOT put your conditional expressions inside quotes.

if("marks >= 95"){

becomes

if(marks >= 95)
{

Since you're a beginner, I HIGHLY suggest you put the opening curly brace on it's own line so you can more easily keep track of which braces line up with each other.

if (marks >= 95)
{
    printf("Grade is A+ \n");
}

To add to what has been said, you almost certainly have a tool which would help you: it's called a debugger. How you use it depends on your compiler system, but a quick Google for the name of your IDE and "debugger" should give you the info you need. What it does is allows you to execute your code line by line and look at variables to see what they contain, and follow where the code goes while it is running.

Put a breakpoint on the first line in the function, and run your code through the debugger. Then look at your code, and at your data and work out what should happen manually. Then single step each line checking that what you expected to happen is exactly what did. When it isn't, that's when you have a problem, and you can back-track (or run it again and look more closely) to find out why.

Sorry, but we can't do that for you - time for you to learn a new (and very, very useful) skill: debugging!

Two final notes.
Your conditions don't cover all cases (e.g., what happens if marks is 80 ?) and redundant (in the second if, for instance, you don't need to check for marks less than 95).
I would write the code this way:

#include <stdio.h>

int main()
{
  int marks;
  printf("Please enter the marks:");
  scanf("%d", &marks);
    
  if(marks >= 95)
  {
    printf("Grade is A+ \n");
  }
  else if(marks >= 80)
  {
    printf("Grade is B+ \n");
  }
  else if(marks >= 70 )
  {
    printf("Grade is B \n");
  }
  else 
  {
    printf("Grade is C \n");
  }

  printf("Thank you!");
  return 0;
}