Which word best completes the sentence? en los restaurantes de ecuador, la gente fumar en el exterior. necesario ni siempre debe

In Spanish (castilian), the verb deber (to owe) means to have a debt to someone.

However, the verb debe  have another meaning:

This acts as an auxiliary verb (similar to English), being written before the main verb of the sentence. This use of the verb debe is called verbal periphrasis or verbal phrase, and generally is written in a sentence as follows:

And, for this case the meanings are as follows:

a) deber: implies obligation or need, but can also express assumption or belief (the first option is preferred).

Example: Ana debe estar en la escuela

Ana must be at school

This may mean:

1. It is mandatory that Ana is in school

2. I guess Ana is in school

b) Deber de:  only expresses assumption or belief.

Example: Ana debe de estar en la escuela

Ana must be at school

It means that:

I guess Ana is in school

Now, in the following sentence, the main verb fumar (to smoke) is in the infinitive and with the 3rd person in singular la gente (the people). From there it follows that the auxiliary verb debe must be used.

Then the complete sentence is:

In restaurants in Ecuador, people must smoke outside.

Indicating that it is an obligation that people smoke outside.

The other options are incorrect because they do not fit in the context of the sentence:

Necesario means necessary

ni means neither

siempre means always