Final answer:
Phrenology faced major issues due to lack of scientific evidence, relying on skull shapes to define personality traits and mental abilities, which was eventually discredited.
Step-by-step explanation:
The major problem with phrenology is A) Lack of scientific evidence to support its claims. Phrenology, which focused on the shape and bumps of the skull, alleged that these physical characteristics correlated with different personality traits, behaviors, and mental abilities. This early form of psychology, popularized in the 19th century by Franz Joseph Gall, assumed that the brain had 37 different 'faculties' that could be measured through the cranium's shape. This became a widespread practice and was even used as entertainment. However, it was ultimately discredited as it lacked empirical evidence to substantiate its methodologies and claims. Phrenology is now considered a pseudoscience, and modern neuroscience has entirely refuted the notion that the shape of the skull is indicative of personality or cognitive abilities.