To understand the meaning behind the statement "Only real friends will tell you when your face is dirty," we need to analyze the options provided:

A. Only a real friend will have guts to say that your face is dirty when it is not.

This option implies that a real friend would falsely accuse you of having a dirty face. However, this contradicts the meaning of the statement, which suggests that a real friend would tell you when your face is actually dirty, not when it is not.

B. Only when somebody doesn't know you, he/she can criticize you. A real friend just cannot do that.

This option suggests that only someone who does not know you well would criticize you. However, the statement is not about criticism but rather about a real friend's honesty in telling you when your face is dirty.

C. Only a real friend will tell you the truth.

This option aligns with the meaning of the statement. It suggests that a real friend will be honest and tell you when your face is dirty.

D. A real friend will only tell you the truth when he/she is sure that you can take it.

This option adds a condition to when a real friend would tell you the truth. However, the statement does not specify any conditions but rather emphasizes that a real friend would tell you when your face is dirty.

Based on the analysis, option C, "Only a real friend will tell you the truth," best captures the meaning behind the statement. A real friend is someone who is honest with you, even when it may be uncomfortable or difficult to hear.

Therefore, the correct answer is: C. Only a real friend will tell you the truth.