Final answer:
The expression √2 - √6 is already in its simplest form and cannot be combined further since they are square roots of different numbers. Option (a) is correct as stated.
Step-by-step explanation:
The student's question is asking to simplify the expression √2 - √6. To simplify this expression, we cannot actually combine these two terms as they are irrational numbers with different radicands. The expression is already in its simplest form. Therefore, the answer is simply a) √2 - √6. The other options presented in the question are not simplifications of the given expression but rather different expressions entirely.