We need to find a common divisor for all the given numbers. Let's test the options: For $7$: $161 \div 7 = 23$, $371 \div 7 = 53$, $301 \div 7 = 43$, $637 \div 7 = 91$, $553 \div 7 = 79$. Since $7$ divides all the numbers exactly, it is the correct answer.