The correct answer is "are". In English grammar, we need to take into account subject-verb agreement.
In this case, the noun "workers" is in the plural so the correct form of the verb "to be" should also be in the plural form (are).
Subject-verb agreement is achieved when the form of the verb "agrees" with the subject. (plural-plural; singular-singular)
The correct sentence reads as follows:
"The workers at the mill are planning to strike."