This doesn't affect the argument. The advertisement only says that Gem World offers written certification and therefore fair prices, not that it is the only store that offers these things. This claim could still be true even if many other stores offer written certification.

This is a tempting answer choice. But just because someone has years of experience assessing gems doesn't mean they can't work in sales for Gem World, and so, by the advertisement's logic, be tempted to say a diamond is higher quality than it actually is. To strengthen the argument, we would want an answer choice that tells us this is not the case.

This doesn't matter. What we want to know is whether Gem World offers fair prices on diamonds, regardless of how high-quality they are.

We're not interested in specific prices, let alone the specific prices for the most expensive diamonds. We want to know if the prices offered at Gem World are fair given the diamonds' quality.

This answer choice tells us that the people appraising the diamonds are independent, and not working in sales for Gem World. This makes it clear that Gem World is different from most other jewelry stores, and strengthens the argument that they offer fairer prices.