Aboard means:

The term "aboard" in the context of equine activities typically refers to being on or in association with a horse, particularly relating to the evaluation or assessment of a horse's qualities and suitability for a certain purpose, such as sale or lease. Therefore, evaluating equines for sale or lease is consistent with the idea of being involved with horses in a professional or transactional manner. This includes assessing their training, temperament, performance capabilities, and overall health, which is crucial for ensuring that potential buyers or lessees are well-informed about what they are acquiring.

The other options do not align with the primary meaning of "aboard." Providing assistance in veterinary care is more about medical support rather than evaluating a horse. Owning and showing horses pertains to ownership and competition rather than the assessment of an equine's qualities. Renting equines for various activities focuses on the leasing aspect rather than the evaluation process involved in deciding what horses are suitable for sale or lease.