Final answer:
The word that means a guarantee is Assure
Step-by-step explanation:
The word that means a guarantee is Assure.
When we assure someone, we are giving them confidence and guaranteeing something. For example, if I assure you that I will be there at 8 pm, it means that you can count on me to be there.
In this case, 'a. Criticize', 'c. Grumble', and 'd. Expose' do not mean a guarantee, so the correct answer is b. Assure.