Which of the following is a valid ground of challenge to the appointment of an arbitrator?

As provided under s. 12 of the Act. 12. Grounds for challenge.- (1) When a person is approached in connection with his possible appointment as an arbitrator, he shall disclose in writing any circumstances,— (a) such as the existence either direct or indirect, of any past or present relationship with or interest in any of the parties or in relation to the subject-matter in dispute, whether financial, business, professional or other kind, which is likely to give rise to justifiable doubts as to his independence or impartiality; and (b) which are likely to affect his ability to devote sufficient time to the arbitration and in particular his ability to complete the entire arbitration within a period of twelve months. (3) An arbitrator may be challenged only if— (a) circumstances exist that give rise to justifiable doubts as to his independence or impartiality, or(b) he does not possess the qualifications agreed to by the parties. (4) A party may challenge an arbitrator appointed by him, or in whose appointment he has participated, only for reasons of which he becomes aware after the appointment has been made. (5) Notwithstanding any prior agreement to the contrary, any person whose relationship, with the parties or counsel or the subject-matter of the dispute, falls under any of the categories specified in the Seventh Schedule shall be ineligible to be appointed as an arbitrator: Provided that parties may, subsequent to disputes having arisen between them, waive the applicability of this sub-section by an express agreement in writing.