What stage do grubs go through prior to becoming adult beetles?

The stage that grubs go through prior to becoming adult beetles is the pupa stage. In the life cycle of beetles, after the grub (also known as the larval stage) matures, it undergoes a transformation into a pupa. During this pupa stage, the grub undergoes significant changes as it develops into an adult beetle. This stage is critical for metamorphosis, where the structure of the insect is reorganized.

The other stages mentioned in the choices are not correct in this context. The caterpillar option is misleading because that term is specifically used for the larval stage of butterflies and moths, not beetles. The egg stage is where the life cycle begins, prior to any development into grubs or larvae. While the larval stage is an important part of the life cycle, it precedes the pupa stage rather than comes after it. Thus, the pupa stage is the correct answer as it directly precedes the emergence of adult beetles.