Which phrase signals a new phase in seriation within a letter?

The phrase that indicates a new phase in seriation within a letter is "Secondly." This term explicitly denotes a transition to the next point or step in a sequence, usually following an initial statement or point introduced earlier. In the structure of a letter, particularly one that aims to present information in a clear and organized manner, using words like "Firstly," "Secondly," and "Lastly" helps the reader follow the sequence of ideas or points being made.

The other phrases do not serve the same purpose of indicating a transition between ideas or points. "On consultation today" is more about the context of a particular meeting or discussion and does not suggest sequencing. "I would be grateful if you could" is a polite request and does not signal a transition. "Her social history suggests" introduces specific information related to the subject but does not imply a movement to a new phase in an organized sequence. Thus, "Secondly" is the clear choice for indicating a new phase in seriation.