A incurs a debt to a firm of which B, C and D are partners. B is insane and C is a minor. D can give a…

Time runs against all the partners because a valid discharge can be given without the concurrence of those under disability. The statute treats that ability to discharge the common claim as decisive for all jointly entitled persons.

Why not "Only against D"? The disability rule for joint claimants does not limit the running of time to the partner personally free from disability. Where that partner can give an effective discharge without the others, time runs against everyone jointly entitled. This alternative incorrectly separates the common claim into individual limitation outcomes.

Why not "Only against B and C"? This reverses the effect of the statutory rule. The partners under disability are not the only people against whom time runs, while the capable partner is excluded. The stated power to discharge the debt without their concurrence causes limitation to run against the whole jointly entitled group.

Why not "None of the following alternatives: only against D; only against B and C; or against B, C and D."? The choices contain the result expressly required by the joint-disability provision, so none of the following is not appropriate. The facts specify a valid discharge without concurrence from the disabled partners. That circumstance makes time run against all three rather than leaving the question without a correct alternative.