The answer is D. First, students must fill out their health forms completely. Then, they must head to the gym for orientation.
Step-by-step explanation:
In the sequential organization, the details included are part of a process, sequence or set of steps; due to this, the sequential organization can be identified by the use of words such as first, finally, then, after or similar that describe a set of stages in order. This can be seen in the case of "First, students must fill out their health forms completely. Then, they must head to the gym for orientation" because this describes a process, also the use of the words "first" and "then" that show order or sequence prove this sentence follows the sequential organization.