C does not have a dedicated boolean type in its original standard; instead it treats integer (and pointer) values as truth values in a condition. The rule is simple and fundamental: the value 0 is interpreted as False, and any non-zero value is interpreted as True. This applies to the controlling expression of if, while, for and the ternary operator alike.

Consequences of this rule are that if(5), if(-1) and if(0.1) all take the True branch because none of these values is zero, whereas if(0) takes the False branch (or the else, if present).

Evaluating the statements:

• A non-zero value in the condition means True — this is the correct statement and a direct restatement of C's truth rule.
• The claim that a value of 0 means True is the reverse of the actual convention; 0 means False.
• The claim that every if must have a matching else is wrong: the else clause is entirely optional, and a lone if is perfectly valid.
• The claim that if statements cannot be nested is also wrong; nesting one if inside another (or inside an else) is common and fully supported.

Hence only the statement that a non-zero condition value means True is correct.