I think the invitations ____ (should / send) by registered post.
should send
should be sent
should sended

employment - employable, retirement

ambition - unambitious, ambitiously

to secure - secure, securely

strong - strength, strenghten

proof - provable, theory

to train - trainable

qualification - to qualify, qualificative, disqualification

defenceless - to defend, defendable

to entertain - entertainable

science - art, scientifically

remarkable - to remark, unremarkable

to vary - to unvary

to enjoy - enjoyable, to dislike

broadly - broadness, to broaden, broad, narrowly

neighbour - neighborly

to respond - to ask

honest - honesty

help - to help, to harm, helpfully

decoration - decorative

confidently - confidence, confident, to confide, uncertainly

applicant - to apply

harm - to harm, to help, harmfully (harmlessly)

bright - brightness, to brighten

tolerant - to tolerate, intolerant, tolerably

to rely - reliable, reliability

to lead - leading, leaderly

high - highness, to heighten, low

dream - to dream, dreamless, reality

play - playful, work

profession - to profess,  avocation, professionally

particular - particle, to impart, imprecise

nation - to nationalize, foreign, nationally

popular - popularization, popularly