In simple terms a covalent bond is when we have sharing of electrons, which means there is no polarity being formed when the elements are bonded, this is more likely to occur with atoms with high electronegativity, or same pair of elements, like O2, where we have O - O (Oxygen bonded with Oxygen), since they have the same electronegativity, there is no charge being formed
The same can not be told for ionic bonds, where we have very different elements being bonded together, with a big electronegativity difference, which means we have polarity being formed (which basically means that we have charge in the compound)
One way to find out if a compound is covalent or ionic is by checking their positions in the periodic table, for example, let's analyze our question
We have:
A) Fluorine - Fluorine, here we have a classical example of covalent bond, since there is no "stronger" element, pulling the electrons towards its side, they are the same, hence, the same strength
B) Carbon - Chlorine, we have two non-metals, with a relatively high electronegativity each one of them, so there is ions being formed, therefore it is covalent
C) Potassium - Oxygen, in this case we have a metal and non-metal bonding together, and since potassium has only 1 electron in its valence shell, it will easily give up this electron to another element, therefore K will receive a positive charge (which means loss of electron) and oxygen will receive a negative charge (which means gain of electron), and this is an ionic bond
D) Oxygen - nitrogen is the same case as letter B
It is also possible to check an electronegativity table, in which there are some rules that helps you understand if a compound is more likely to be covalent or ionic