Convert the decimal to a percent.

3.45

a. 345%

b. 34.5%

c. 3.45%

d. 0.345%

In converting a decimal to a percent you just need to multiply it by 100. 
Solution on the problem above:

3.45 x 100 = 345% 

Therefore the answer is letter A. 

It will be vice versa in converting percent to decimal. You just need to divide it by 100. 
For example, we convert the 345% to decimal:
345%/100  = 3.45