Updated 12-03-2016
Isomer(s) of a molecule are defined as molecules that have the same molecular formula, but have a different arrangement of the atoms in space
Here we see ...the same molecular formula... but different arrangement. Now lets take your example; pentane has the chemical formula $\ce{C5H12}$
but it has a few isomers, but the molecule 2,2 dimethylpropane has the same chemical formula as pentane ($\ce{C5H12}$), the difference being in their structure. So is the formula for 2 methylbutane.
These molecules having the same formula but different structure are said to be isomers of one another, As a group of isomers of ($\ce{C5H12}$) they are reffered to as the isomers of pentane which includes n pentane So When the word isomer of something is mentioned, it will include the something and it's isomers. It's like an album with the thumbnail from the same album.
If you say isomers of 2,2 dimethylpropane it will include n pentane as well as neo pentane and iso pentane and 2,2 dimethylpropane.
. In the above case 'C' the pentane is n-pentane or normal pentane with no branches[ an isomer of pentane (or what you may refer to as the pentane itself, as per simple IUPAC nomenclature of alkanes) ].iso pentane also called as 2 methylbutane is an isomer of pentane as well. neo pentane or 2,2 dimethyl propane is also an isomer of pentane.
D in above case is identical to pentane in terms of carbon number, even in terms of general structure but has one less hydrogen, so the chemical formula is not same as it belongs to a different organic class of compounds (alkenes) and thus is not an isomer of pentane.