Answer :

1:200 is the strongest

I don't know what you mean by the 'strongest', but 1:10 is the greatest in this set of ratios.

because

1:10 is 1 divided by 10, 1:100 is 1 divided by 100, and 1:200 is 1 divided by 200.

if it's divided by a large number, than the number decreases.