D is the correct ratio. If his heart beats 12 times in 10 seconds, then it will beat 72 times in 60 seconds/1 minute if you use the proportion. D is the only correct ratio because it shows 72 beats:1 minute.
To estimate this I find it easy to turn the fractions into decimals and then you can easily estimate. To turn a fraction into a decimal divide the numerator by the denominator.
