Clinical practitioners ___________ integrated mindfulness _____________ treatment of ________ host of emotional and behavioural disorders, ________ borderline personality disorder, major depression, chronic pain, or eating disorders. Number of such practitioners _________ increased substantially.

In the given sentence, treatment should be preceded by 'the' as we are talking about particular treatment for given disorders. Therefore, options C and D are eliminated. In the given sentence, number of such practitioners is followed by a singular verb, i.e. has is used instead of have.

The use of "have" in the first blank is appropriate because "Clinical practitioners" is a plural subject, and the present perfect tense ("have integrated") indicates an action that started in the past and continues to be relevant in the present.

The answer is option A.