Explanation
Correct answer: D.
Choice D is the best answer. "Next" logically signals that the information in this sentence—which explains how Celado builds the pot’s walls—is the next step in Celado’s pot creation process. The sentences that follow further signal this information by completing the process.
Why the other choices are wrong
Choice A
Choice A is incorrect because "for example" illogically signals that the information about Celado building pot walls in this sentence exemplifies the information in the previous sentence about how Celado’s pot creation process begins. Instead, it is the next step in this process.
Choice B
Choice B is incorrect because "however" illogically signals that the information in this sentence contrasts with the information in the previous sentence about how Celado’s pot creation process begins. Instead, it is the next step in this process.
Choice C
Choice C is incorrect because "by contrast" illogically signals that the information in this sentence contrasts with the information in the previous sentence about how Celado’s pot creation process begins. Instead, it is the next step in this process.