Select the sentence that correctly uses commas in a non-restrictive phrase.
a a river, that is polluted, is unsafe for swimming.
b polluted rivers, which can exist anywhere, are unsafe for swimming.
c when a river becomes polluted, it is no longer safe for swimming.
d however, rivers sometimes become polluted when nearby farms use too many poisons.

Your answer would be B.

Explanation:

The sentence that correctly uses commas in a non-restrictive phrase is sentence B. This sentence contains the nonessential relative clause which can exist anywhere.

Relative clauses, also called adjectival clauses, function as adjectives. They add description to the sentence; they modify nouns mainly. In this case, the clause modifies the noun rivers. Moreover, the clause is non-restrictive because the information it provides is not considered essential to complete the meaning of the sentence. That is, if the clause was dropped, the meaning of the sentence would be left unchanged (see 1).  For this reason, the clause is also offset with commas.

1) Polluted rivers are unsafe for swimming

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