If you haven't made a valid choice after five seconds, Modular Monstrosity loses all keyword abilities, including ones its picked up in other ways.

If you make an invalid choice, you can still make a valid choice as long as time hasn't expired.

You may choose any keyword ability that appears on any printed Magic creature card. You can't pick keyword actions like explore or scry. If the keyword has a modifier, such as a mana cost or a number, you must choose a modifier that's been printed. Sorry, "annihilator 500" fans.

Each keyword can be chosen only once on any given day. And that counts every Modular Monstrosity you play with that day. You also can't reuse a keyword with a new modifier. For example, if you choose protection from Demons and from Dragons (totally counts), you can't choose protection from red later that day. Protection has already been used.