The RNA polymerase will read the strand with the help of a base, which pares codes for m-RNA. The pairs are adenine (A) with thymine (T) and guanosine (G) with cytosine (C). But no thymine is present in RNA, as it is replaced by uracil (U).
Therefore, you must pair A with T and G with C, and replace T by U after that, as follows:
Original strand: ATGCTA
Paired: TACGAT
mRNA: UACGAU (final answer)