To determine which compound gives a secondary alcohol upon reaction with methylmagnesium bromide (CH3MgBr), we need to understand the mechanism of the Grignard reaction and how it leads to the formation of alcohols.

The Grignard reaction involves the nucleophilic addition of a Grignard reagent (an organomagnesium compound) to a carbonyl group, resulting in the formation of an alcohol. In this case, methylmagnesium bromide (CH3MgBr) will act as the nucleophile and attack the carbonyl group of the given compound.

Let's analyze each compound and see which one is most likely to give a secondary alcohol:

a) Butyl formate: This compound contains a formate ester group (R-COO-R'), where R is a butyl group. When the Grignard reagent reacts with the carbonyl group in butyl formate, it will form a tertiary alcohol. However, we are looking for a secondary alcohol, so this compound is not the answer.

b) 3-pentanone: This compound is a ketone, and ketones react with Grignard reagents to form secondary alcohols. Therefore, it is a potential candidate.

c) Pentanal: This compound is an aldehyde, and aldehydes also react with Grignard reagents to form secondary alcohols. Therefore, it is another potential candidate.

d) Methyl butanoate: This compound is an ester, and esters react with Grignard reagents to form tertiary alcohols. Therefore, this compound does not give a secondary alcohol.

Based on the analysis above, the correct answer is option 'c' (Pentanal). When pentanal reacts with methylmagnesium bromide, it will undergo nucleophilic addition, resulting in the formation of a secondary alcohol.

Overall, understanding the reactivity of different functional groups towards Grignard reagents allows us to predict the products of their reactions and determine which compound will give a secondary alcohol in this case.