Which style rule would make the image 50% smaller during a hover?

Which style rule would make the image 50% smaller during a hover?
<img id=”photo” alt=”” src=”…” />

• img#photo:hover {scale: 0.5;}
• img#photo:hover {transform: scale(0.5);}
• img#photo {hover-scale: 0.5;}
• img#photo:hover {size: smaller;}