Hello!
The answer is: Cincuenta por ciento.
Why?
In Spanish, like English, we can write percents by using number and symbols (%) or just using words.
In Spanish we can have two situations that will determinate the way we should write the words referring to a percent:
The first situation, when we know or we will mention the percent value: We can write it like "por ciento" or we can use the percent symbol (%)
The second situation, when we don't know or we won't mention the percent value: We should write it like "porciento" and we shouldn't use the percent symbol (%)
Have a nice day!