What does 'non-credible' mean? Why don't we use non-refundable in this case?

[Orchid1205]

English Language Tests, Intermediate level

ESL/EFL Test #725 "Real Life: Apartment Hunting (1)", question 2

A landlord might ask you for a ......... deposit in order to hold an apartment for you. This means that if you decide that you don't want to rent that specific apartment, you won't get the deposit back.

(a) non-refundable
(b) non-insurable
(c) non-credible
(d) non-fat

English Language Tests, Intermediate level

ESL/EFL Test #725 "Real Life: Apartment Hunting (1)", answer 2

A landlord might ask you for a non-refundable deposit in order to hold an apartment for you. This means that if you decide that you don't want to rent that specific apartment, you won't get the deposit back.

Correct answer: (a) non-refundable

Your answer was: incorrect
A landlord might ask you for a non-credible deposit in order to hold an apartment for you. This means that if you decide that you don't want to rent that specific apartment, you won't get the deposit back.
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What does "non-credible" mean? Why don't we use non-refundable in this case?
Please tell me know how to use "non" as a prefix. What does it mean?

[Kitosdad]

A none refundable deposit is asked, because the owner could have rented the apartment to another person in the time that he waited for you to decide.

Non-credible would mean without credibility. For example, a known criminal would not be afforded the same credibility as a known honest person during a dispute, or court case.

Kitos.
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[Orchid1205]

thank you for your explaining, Kitos.