Concept:Short replies like “So do I” are used to show agreement with a positive statement.
Explanation:Tayo’s sentence is “I think I can now solve the problem.”
The main verb is “think,” not “can.”
So when Essien agrees, the reply must match the auxiliary of the main verb “think.”
For a positive present simple verb, we use “do” in the short agreement form.
Therefore, “So do I” means “I think so too.”
“So can I” would only fit if Tayo had said “I can solve the problem.”
Answer:D. So do I