1. What do the following words have in common?

AD
SUM
CELL
COMB

2. What do the following words have in common?

VICE
BALL
DRESS
OPTION

Answers:

Each word can be changed into a new one by adding the letter O at the end:

ADO
SUMO
CELLO
COMBO

2. Each word can be changed into a new one by adding AD at the beginning or end:

ADVICE
BALLAD
ADDRESS
ADOPTION