This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Techniques of Ocular Examination and Diagnostic Tests, Subject name: Ophthalmology
The instrument shown below is: 
Select and explain. 
A. Maddox rod, B. Retinoscope, C. Pachymeter, D. Placido disk
D: Placido disk
Answer: D: Placido disk 
Explanation: Ans. (d) Placido disk.