In each of the following two sets I & II, find the word or pair of words different from the other three words or pair of words:
I: J. Lake K. Brook
L. Stream M. River
II: J. Weighty-Heavy K. Broad-Wide
L. Big-Large M. Tiny-Small

In I, for all the options excluding lake, the water body is not stagnant and keeps on flowing whereas, the water in a lake remains stagnant and hence is different from the group.

In II, for all the pair of words except Weighty- Heavy, the second word signifies something that can be physically measured. for example, we use wide for physical width of an object. Similarly Big and Small are used to signify the physical size. But in Weighty-heavy, it is the first word that makes sense when we talk in terms of physical measurements. 

Therefore, Option A.