Finding and replacing lines that begin with a pattern

I have a text in a file file.txt like this

xxxxxxxxxxxxxxx
xxxxxxxxxxxxxxx
a    b   c // delimited by tab
xxxxxxxxxxxxxxx
xxxxxxxxxxxxxxx

I know using sed I can find and replace text in a file. If a line starts with a b(seperated by a tab) I need to replace it with d e f. So the above file will be

xxxxxxxxxxxxxxx
xxxxxxxxxxxxxxx
d    e   f // delimited by tab
xxxxxxxxxxxxxxx
xxxxxxxxxxxxxxx

I can do this to find and replace, I want only those instances where the line starts with a b and replace the whole line.

sed -i 's/a/\t/\b/\t\/c/REPLACED TEXT/g' file.TXT