Not necessary, because there can be other equally effective means of preventing inflation/deflation. For example, both money and goods/services could grow at the same rate. The author doesn’t have to assume that stability is the best way to prevent inflation/deflation.

Not necessary, because the author can believe there are other ways besides gold for a stable money supply. Although the author believes that having money anchored to gold is sufficient for the supply to stable, that doesn’t mean he believes its necessary.

Necessary, because if it weren’t true — if the production of goods/services in the economist’s country is likely to grow markedly — then deflation would occur (because deflation occurs when production of goods/services grows more than money supply grows).

Not necessary, because even if inflation is slightly more likely to occur than deflation, the economist can still try to prove that both are unlikely to happen. Maybe inflation has a 5% chance of happening, and deflation a 4% chance — you can still argue that both are unlikely.

Not necessary, because there can be other equally effective means of preventing inflation/deflation. For example, both money and goods/services could grow at the same rate. The author doesn’t have to assume that stability is the best way to prevent inflation/deflation.