You want to merge two dataframes $($df$1$ and df$2).$ Each dataframe has a column called $$idval$$ which can be used as a key. Which part of the statement:

dffinal $=$ df$1.$merge$($df$2,$ how$=$'left', on$=$'idval'$)$

suggests which dataframe is on the $$left$$ of the merge $($join$)?$

a$.$

how$=$'left'

b$.$

df$1$

c$.$

dffinal

d$.$

df$2$