Final answer:
An increase in the normal strain of an object, with all other factors held constant, can be caused by changes in its length, as strain is related to the deformation resulting from applied forces according to Hooke's law.
Step-by-step explanation:
If all other factors remain constant, an increase in the normal strain of an object can be caused by changes in its length. Deformation, which is the change in shape due to the application of force, strongly relates to strain. In terms of Hooke's law, deformation is proportional to force. Therefore, if an object is subjected to increased tensile or compressive forces, resulting in a change in its length, this will lead to an increase in normal strain. Environmental factors such as temperature can also affect strain, but these are not included in the list of options provided.