too many rows on wp_options table

On one of my sites I saw that in wp_options table there are more than a hundred thousand rows
after a while they all start similar to something like:
_wp_session_
_wp_session_expires_
and it mosty sas things like “you must anable payment gateways to use digital downloads” etc….

it looks like someone inserted this bad content into my database.

Question 1) Is it ok if I just delete these rows or it will harm something?
Question 2) How can I prevent someone inserting things into my database again? and my password was strong anyway
Question 3) what were they doing with my database? I didnt notice anything visible on my site

my most importnat question at this point is question 1