The banana that contains high level of potassium and other important

The OA is B.

This sentence discusses characteristics of “the banana,” a singular subject. Thus, the plural verb “are” is incorrect. Additionally, the phrase “considered to be” is unidiomatic. In standard written English, “considered” is accompanied by neither a preposition nor a verb. Finally, the relative pronoun “that” makes it seem that a particular banana, rather than bananas in general, contains high levels of potassium.

(A) This choice is incorrect as it repeats the original sentence.

(B) CORRECT. This choice correctly uses the singular verb “is” with the singular subject “banana,” and correctly uses “considered” without an accompanying preposition or verb. Additionally, the relative pronoun “which” properly introduces a non-restrictive clause that indicates all bananas, rather than one specific banana, contain high levels of potassium.

(C) The plural verb “contain” is incorrectly paired with the singular subject “banana.” Additionally, “considered to be” is unidiomatic.

(D) The relative pronoun “which” is incorrectly used to introduce a restrictive clause. Also, the sentence seems to indicate that certain bananas, rather than all bananas, contain high levels of potassium.

(E) This choice incorrectly uses the unidiomatic “considered to be.”

shouldn't "which" be preceded by "," ?
< asking for "D" >

The sentence at the end says fruits. The part with banana is underlined that cannot be used to determine the singularity or plurality of the sentence.
The answer in this case should be (d). Bananas are considered to be one of nature's healthiest fruits.

I agree with you. I too selected options D for the same reason.
Can anyone put some light on that.