I am unsure of whether to use singular or plural describing a list where the objects in the list are choices

To me they mean different things:

Three different strategies whereby X and Y are produced in a ratio of 1:2, 1:1, or 2:1. > (i) X and Y are separate products made of A and B. the ratio refers to the A:B relationship.

Three different strategies whereby X and Y are produced in ratios of 1:2, 1:1, or 2:1. > X and Y are the components of a single product. “Ratios” refers to the X:Y relationship.

Then there is   Three different strategies whereby X and Y are produced in a ratio of either 1:2, 1:1, or 2:1. > (i) X and Y are separate products that occur or are produce as a result of the variables of a system.

First, if those are sentences, neither of them is correct. They are both sentence fragments.

Look at the essential component of the sentence:

 ？ Three different strategies whereby X and Y are produced.

That's syntactically the same as the following:

？ Three different people who walk down the street.

Both of those are noun phrases, but there is no main verb. What do the strategies do? What do the people do?

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That aside, the question doesn't make it clear if each strategy produces X and Y in a different ratio, or if they simply produce X and Y in a ratio that matches one or more of the three ratios.

Possible wordings include the following:

• Each of the three strategies produces X and Y in a unique ratio of 1:2, 1:1, or 2:1.
• Each of the three strategies produces X and Y in a ratio of 1:2, 1:1, or 2:1.
• The three strategies respectively produce X and Y in ratios of 1:2, 1:1, and 2:1.
• The three strategies produce X and Y in a ratio of 1:2, 1:1, or 2:1.

Note the switch from the indefinite article to the definite article in the one case, because all three are described at once, and each must use a unique ratio.