To determine how many pens Kate has based on the information provided, we know that Kate has 5 times as many pens as Mark. Since Mark has 10 pens, we can express this relationship as:
\[ y = 5 \times 10 \]
So the correct equation that answers how many pens Kate has is:
\[ y = 5 \times 10 \]
This means Kate has 50 pens.