Subwords means letters must be consecutive as in the word.
Since we have 6 letter word NAMITA therefore we will have
6 one letter subwords (i.e. N,A,M,I,T,A) but out of these one A is repeating at last therefore we neglect it as it says distinct letter subword.
Hence we have 6-1=5 distinct one letter subwords(i.e. N, A,M,I,T) of the word NAMITA.