A period is a horizontal row in the periodic table. Although groups generally have more significant periodic trends, there are regions where horizontal trends are more significant than vertical group trends, such as the f-block, where the lanthanides and actinides form two substantial horizontal series of elements.

Based on this passage, this is a horizontal arrangement of elements in the periodic table? Let's give stream of consciousness first:
To answer the question, consider the following: A period is a horizontal row in the periodic table.
So, the final answer is A period.