Which sentence uses an objective tone?
we will plant the best garden of any school in the area.
the new campus garden will surely be envied by other schools.
a garden would provide numerous benefits to the students.
you will probably agree that gardens have many benefits.

Objective tone is used to provide unbiased, neutral, factual information. Therefore, items written using an objective tone are often impersonal and do not include any opinion or information whatsoever about the writer. Within this list, the sentence that meets with all these requirements and uses an objective tone is: a garden would provide numerous benefits to the students.

Objective tone refers to a style of writing which is impersonal. An objective tone doesn't include the information about the author but about something. For instance,

The dish took 15 minutes to cook.

It is an objective tone

Whereas, a sentence such as:

I took 15 Minutes to cook the dish.

It is a subjective tone as it includes 'I.'

Therefore, sentence which uses an objective tone is:

A garden would provide numerous benefits to the students.

you need to include the text. we have no idea what you are referring to

hey! what was the rest of the question?