1.

a.

Iterate over each character in the sequence from the beginning to end, replacing each A, T, G, and C with its T, A, C, and G, respectively.

b.

No!

c.

def complement(sequence):
    """ (str) -> str

    Return the complement of sequence.

    >>> complement('AATTGCCGT')
    'TTAACGGCA'
    """ 

    complement_dict = {'A': 'T', 'T': 'A', 'C': 'G', 'G': 'C'}
    sequence_complement = ''

    for char in sequence:
        sequence_complement = sequence_complement + complement_dict[char]

    return sequence_complement