Final answer:
Direct High Frequency treatments are best suited for oily or acne-prone skin as they help to reduce excess sebum and have antiseptic effects.
Step-by-step explanation:
A Direct High Frequency treatment is best suited for skin that is d. oily or acneic. Direct High Frequency treatments help dry out excess sebum and have an antiseptic effect on the skin, which is beneficial for oily or acne-prone skin. In contrast, a) dry skin can be treated with topical moisturizers, b) normal skin can result from deep massages, and c) aging skin is preventable by eliminating pressure points. Dry skin (d) is not directly caused by direct high-frequency treatments, but such treatments are not generally recommended for dry skin as they might exacerbate dryness.