In the following question a certain relation exists between the first two words. The same relation also exists between the third and the fourth word. Out of these four words one word is missing and is denoted by a question mark in the question. Find out the missing word from the words given below.
Metre : ? :: Litre : Volume

Length

The correct answer is Option 3: Length.

Here's why: The relation between the first two words is that they represent units used to measure specific properties.
    Metre is a unit of length.
    Litre is a unit of volume.