The clause, 'It is important that' indicates that the clause which follows expresses a wish or a command that something be done. The action which is expected to be performed (usually by the third person noun or nouns) is said to be in subjunctive mood. Subjunctive mood expresses something that one wishes for, or a hypothetical rather than an actual situation. The verb in subjunctive mood is always in present tense without the letter 's'. 

In the given sentence, there is an expectation / wish 'that all employees need to be dressed in proper uniforms'. The verb in subjunctive form is 'be'. It is expressed in the simple present tense form i.e. 'be'. Hence, the correct answer is 'be dressed' as given in option A. 

Given the format of the verb in subjunctive mood (i.e. present tense without the letter 's'), options B and C are incorrect.

Option D is incorrect as option A is the correct answer.