The phrase, 'desirable that' is followed by the present subjunctive form, i.e., the base form of the verb. So, 'furnish' will be correct here. Option b is ruled out as it uses the m odal 'should' (obligation) which is not needed in this context; c is ruled out as 'it is highly to be desired' is an incorrect expression; d is ruled out as 'should'(obligation) and 'have to'(necessity) are separate modals that give confusing sense if used together.