The correct option is
A applies
In order to replace the underlined part of the sentence with the correct form of the verb, we must understand the tense of sentence.The given sentence is a general statement as it explains us about a rule. The word 'this' explains that the sentence is in present tense. Hence, we will replace the underlined sentence with simple present tense of the verb.
Option B - 'Applying' is the participle form of the verb 'apply', which is used for an action happening at a given time. It is always accompanied by a helping verb or an auxiliary. As the given sentence is a general fact, we need the verb to be in the 'simple present tense' form. Hence, option B is incorrect.
Option C - 'Has applied' is the present perfect tense and we need simple present tense of the verb. Hence, option C is incorrect.
Option D - As 'rule' is a singular subject, we can't use a plural form of the verb 'apply'. A singular subject requires a singular verb only. Hence, option D is incorrect.
Option A - 'Applies' is the third person form of the verb 'apply', which agrees with the third person subject 'rule'. It is in simple present tense. Hence, option A is the correct answer.
'This rule applies to foreign workers in India.'