1.
If someone picks on you, they repeatedly criticize you unfairly or treat you unkindly. (INFORMAL)
Bullies ~ younger children...
PHRASAL VERB: V P n
2.
If someone picks on a particular person or thing, they choose them, for example for special attention or treatment. (mainly BRIT)
When you have made up your mind, ~ a day when you will not be under much stress...
= pick
PHRASAL VERB: V P n