In each of the two, three, or four-word sets below, the words can be changed into new words which are in the same category. The changes are made by adding or removing a single letter and then, except in one case, re-arranging the resulting letters. The changes may or may not be the same within each set, but in every case the change will involve a single letter.

1)
RULE
LUCID

2)
BOON
DONA
BACON
RANG

3)
LINT
ANGER

4)
RAGES
REIGN
VELCRO

Answers:

1. Add an E to each word to get the names of two mathematicians:

RULE + E = EULER
LUCID + E = EUCLID 

2. Add R to the first three words and O to the fourth to get the names of four elements:

BOON + R = BORON
DONA + R = RADON
BACON + R = CARBON
RANG + O = ARGON

3. Add an A to the first word and a G to the second to get the names of two languages:

LINT + A = LATIN
ANGER + G = GERMAN

4. Remove an R from the first two words and add a G to the third word to get names of three spices/herbs:

RAGES - R = SAGE
VELCRO - R  = CLOVE
REIGN + G = GINGER