To prevent invalid data, computer shows:

by

A. Red triangle
B. Green triangle
C. Blue triangle 
D. Black triangle

🧠 Explanation (Math):

Given: UI indicator used to prevent invalid data
Step 1: Data-entry systems show on-screen visual cues when validation fails
Step 2: The indicator icon/color flags fields needing correction before accept
Step 3: The referenced indicator in this question is the blue triangle cue
Final Answer: Blue triangle