Consider the explanations given below to answer the tricky question.
The concluding statement was ( Math+ program during 3 - 5 years age is responsible for higher score in 3rd and 4th grade).
We are testing for weaken...The weakener shows (some other factor is responsible for higher score).

Option A:- Incorrect
Most parents send their children to pre- school for social development and do not have a clear idea about what type of education they want to for their children.
This option is irrelevant and it does not in anyway give strength or weakens the argument. The bottom line here is 'math + is responsible for higher grade'.

Option B:- Incorrect
Cognitive abilities of 3- to -5 year old children are constantly changing. YES they might be changing however, how as their exposure to math + responsible for their high grade in 4th and 5th grade? This option says nothing about that.

Option C:- correct
The children in the pre- schools that were studied had previously been exposed to another math enrichment program.
NO! they were not previously exposed to another math enrichment program. However, there ages at the pre-school they underwent math+ which makes them have higher grades.

Option D:- Incorrect
Children are not really interested in enrichment programs in pre-school. This is irrelevant and does not prove a point.

Option E- Incorrect
The cost factor needs to be specified and established before a large scale program can be undertaken.
This is definitely out of scope of discussion and it neither make reference to the math+ class nor the high grades know 4th and 5th grade.