You can only know by context or by describing it more specifically (e.g. het ontbijt van morgen), the sentence in the exercise can mean both some specific breakfast or breakfast in general. Putting emphasis on hét doesn't really work since this implies it is some important or remarkable breakfast.
No, though both tenses sound the same. 'Ontmoeten' in the past tense would be 'ontmoetten' with two t's. The rule here is that (with regular verbs) you take the infinitive (ontmoeten) and you take away 'en' at the end of the word so you keep the stem (here: ontmoet). Then you add 'te' or 'de', depending on whether the last letter of the stem is one of the letters tkfschp (mnemonic is ' 't kofschip', but without the vowels). If it is one of those letters, you add 'te' (or 'ten' if it's plural). If it's not one of those letters, you add 'de' or 'den'. The last letter here is a 't', which is one of those letters, so you have to add 'ten' to ontmoet, which makes the double 't'.
Another example would be 'borstelen' (to brush). -en = borstel. 'l' isn't one of the letters in 't kofschip, so the past tense is borstelde (singluar) or borstelden (plural). Hope this helped!
Waarom wordt "ontmoetten" fout gerekend? Je kunt toch niet aan de uitspraak horen of het tegenwoordige of verleden tijd is.