This is an interesting language learning assignment. English is strange in that the same series of letters can represent different phonemes (language sounds). In the case of a), you end up with three different sounds from the same four letters. "Ough" can sound like "uff", "oh", and "ow", respectively.
The phonemes associated with "th" in phrase b) include both a hard and soft sound, with the soft "th" letting through a bit more air between the tongue and top teeth. The words "bother" and "brothers" have the harder consonant blend, and the word "both" has the more aspirated sound.
In phrase c), you again have two different phonemes derived from the letter "o". In the words "to", "do", and the second "to", the phoneme is "oo". In the word "go", the phoneme is "oh".
This can be very hard to figure out without hearing a real person say the words. The Cambridge Dictionary website (as well as many other dictionary websites) has a small red speaker icon next to the definitions of words that will play the correct pronunciation of that word when pressed. I've included the link below.