The correct way to declare a variable in C is option 'C': int x.

Explanation:

In the C programming language, variables are declared by specifying the data type followed by the variable name. The data type determines the type of values the variable can hold, and the variable name is used to identify the variable within the program.

Here is the breakdown of the correct way to declare a variable in C:

1. Data Type:
- In the given options, the correct data type for declaring a variable is 'int'. This data type is used to store integer values.

2. Variable Name:
- After the data type, we need to provide a variable name. In this case, the variable name is 'x'. The variable name can be any valid identifier and should be chosen according to the purpose or meaning of the variable.

3. Syntax:
- The syntax for declaring a variable in C is: data_type variable_name;
- In this case, the correct syntax is: int x;

4. Explanation of Incorrect Options:
- Option 'A' (var x) is incorrect because 'var' is not a valid keyword in C for variable declaration.
- Option 'B' (x = var) is incorrect because it assigns the value of a variable 'var' to the variable 'x' without declaring its data type.
- Option 'D' (x = int) is incorrect because it attempts to assign the data type 'int' to the variable 'x' without declaring it first.

In summary, the correct way to declare a variable in C is by specifying the data type (int in this case) followed by the variable name (x in this case). This follows the syntax of data_type variable_name;