Final answer:
B. Measurable. The goal is missing the Measurable principle of the SMART goal.
Step-by-step explanation:
The SMART goal principle that this goal is missing is Measurable.
To make a goal measurable, it should have a criteria or measurement that can be used to evaluate progress. In the case of becoming more flexible, the goal could be made measurable by specifying the number of days per week or the amount of time dedicated to stretching exercises.
For example, a revised goal could be: 'I will become more flexible by August 1st by stretching for at least 30 minutes every day.'