Final answer:
A planet predominantly made of hydrogen compounds, found in the outer solar system, would be considered an 'ice giant' because it contains more of the 'ice' component of compounds like water, methane, and ammonia, as opposed to a 'gas giant' which is mostly hydrogen and helium.
Step-by-step explanation:
A planet made almost entirely of hydrogen compounds would be considered an ice giant. The term "ice" in the context of planetary composition refers not to the physical state of a substance but rather to compounds that form from abundant elements such as oxygen, carbon, and nitrogen, and includes substances such as water, methane, and ammonia, which are common in the outer solar system. In this outer region, gases dominate the larger planets, Jupiter and Saturn, which are referred to as gas giants because of their composition of hydrogen and helium. On the other hand, Uranus and Neptune contain a much higher proportion of these hydrogen compounds, earning them the classification of ice giants. Therefore, the correct answer to the question is d) Ice giant.