Maskin g occurs when a species other than analyte increases or decreases the analytical signal and makes it appear that the concentration is greater or less than the real concentration.

The correct answer is interference.

An interference is a substance, that can prevent the assayed material from being measured. An interference is a substance present in the sample with the effect of altering the correct value of the result, which is usually expressed as concentration or activity, for an analyte. Interferences cause erroneous analytical results.