An acid is identified by its ability to donate protons or H+ ions readily. The addition of protons or hydrogen ions is what makes the solution more acidic and reduces its pH.
In the given cases, HNO3, also known as nitric acid, will be classified as an acid. Whether in solid or aqueous form, nitric acid readily dissociates into hydrogen ion and nitrate ion, thus acidifying the solution.
Covalent bonds are formed by sharing of electrons between two atoms. These atoms are generally of non-metals, as metal atoms tend to donate electrons rather than sharing them. In the given cases, SBr4 and NF3 are both covalent compounds, as they are formed by non-metals and electron sharing is involved. For example, nitrogen shares its 3 valence electrons with 3 atoms of fluorine, each of which shares 1 electrons, and we end up with NF3.
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