@Experts/Others: kindly add/correct below post if something has been missed out in the generalization of such plan-effect arguments.

We have to look for assumption linking which the can strengthen the conclusion that insurer's payouts to medical-service providers will decrease if it implements the reimbursement plan.

We can look at it more like a plan-effect argument
Premise: bacteria around the gum line -> induce preterm labor in pregnant women
-> can cause heart disease by clogging arteries
(PS: do note the causality is in the premise, we cant challenge it because of its occurrence in the premise)

Conclusion: By creating a dental plan for people who are pregnant and people who are prone to heart conditions -> reduce the insurance payouts (otherwise generally done for above medical outcomes)

So we have a plan -> intended effects. So to strengthen it, we need to make sure of the efficacy of the plan
i. identify any problems with the plan (for the desired effect) - address it with solutions
ii. some kind of statistics that the plan is effective

In this context, there is a dental plan in place for the target patients, so what could be the problems with the dental plan:
i. one year check-up is not sufficient to keep the bacteria in check
ii. patients will actually use this plan
iii. to initiate this claim, there is a tedious and time consuming process ..etc..
=> the list could go on, however the general idea remains the same - to plug these (or any other) loopholes with the plan

(A) Dental-hygiene regimens such as brushing or flossing can slow the accumulation of the bacteria-produced substance.
-> it is giving alternate solutions for the intended effect - irrelevant to our conclusion of the efficacy of the plan. It can be tempting to take this option, as it may seem to have the intended effects, but note our aim is not to find alternative solutions to the plan but to question the effectiveness of the plan in question
(a typical trap answer)

(B) Pregnant women typically crave sugary foods, which are proven to contribute to tooth decay.
-> out-of-scope

(C) The risk of developing heart disease is greater for individuals with a family history of heart disease than for those with no such history.
-> out-of-scope (similar type as B)

(D) Individuals are generally less likely to postpone or forego inexpensive or free dental procedures than expensive ones.
-> Yes, makes sense, patients are likely to adopt this plan, thus offering a solution to one of the loopholes that probably plan wouldnt have been adopted

(E) The dental health of pregnant women and heart-disease patients, as a group, are similar to those of the general population.
-> out-of-scope

So what we can generalize about the wrong answer choices for such plan-effect question "Strengthen" type:
Type 1: Answer choices which offer alternate solutions to get the desired effect (irrelevant)
Type 2: Answer choices which offer other effects for the said plan (irrelevant)
Type 3: Out-of-scope choices, which doesn't touch the plan-effect scenario, but talks about certain aspects of the plan or effect (e.g. how one contracts the said conditions, general statistics about the patients)