Which of the following ranks has a simple insignia of a single bar?

The rank that has a simple insignia of a single bar is indeed the Cadet Seaman Recruit. This rank typically represents the entry-level position in the cadet hierarchy. The use of a single bar as insignia signifies that this rank is foundational and indicates the beginning of a cadet's journey in gaining knowledge and skills in maritime service. The simplicity of the bar also reflects the uniformity and functionality often associated with initial ranks in military and naval organizations.

In contrast, the other ranks mentioned have more complex insignias. For instance, the Cadet Petty Officer Third Class typically features a combination of symbols that denote a higher level of responsibility and experience, while Cadet Lieutenant Junior Grade, as a higher rank, has an insignia that indicates significant leadership roles. Therefore, the distinguishing feature of a single bar for the Cadet Seaman Recruit is what makes it the correct choice in this context.