#### Answer

$a.$ and $c.$ are correct.

#### Work Step by Step

To write a decimal as a percent, drop the percent symbol move the decimal point two places to the right and attach the percent symbol, %.
$a.$ is correct because $0.231=23.1$%
$b.$ is incorrect because $5.12=512$%
$c.$ is correct because $3.2=320$%
$d.$ is incorrect because $0.0175=1.75$%