Final answer:
A person's mental abilities are mainly influenced by genetics, while their current mental activity is directly shaped by sensory input, corresponding to choice (c) Brain structure; sensory input.
Step-by-step explanation:
In terms of the brain, a person's mental abilities are determined primarily by a complex interplay of factors including genetics and environmental influences. The best answer to the first part of the question is genetics; this is because an individual's initial capacity for mental abilities, such as intelligence and temperament, can be hereditarily influenced.
A person's current mental activity, on the other hand, is determined primarily by a multitude of factors including sensory input, neurotransmitters, and cognitive processes. However, sensory input is the most immediate determinant of current mental activity as it is the information the brain receives and processes at any given moment, directly shaping our current experience and cognitive state.
Therefore, the correct answers are: a person's mental abilities are determined primarily by genetics; a person's current mental activity is determined primarily by sensory input. This corresponds to answer choice (c) Brain structure; sensory input.