The structures of a protein increase in complexity all the way up to quaternary structure. Primary structure is based on the amino acid sequence of the protein, while secondary and tertiary structures are based on intermolecular attractions between the amino acids in the polypeptide. Quaternary structure is only seen when a functional protein complex is composed of two or more polypeptides bound together.

The correct answer is more than one of these. Primary structure is defined as a succession of amino acids joined by peptide bonds. Secondary structure introduces dimensionality to a protein via hydrogen bonding to produce two predominant structures, alpha helices and beta-pleated sheets. Tertiary structures cause further protein folding by disulfide bonds between cysteines, Van der Waal interactions, and hydrophobic interactions. Quaternary structures involve multiple amino acid chains folding together, and utilize the same types of bonds as tertiary structures. 

The 5' cap is recognized by the important translation factor eIF4e. Once bound, eIF4e helps transport the mRNA molecule to the ribosome and facilitates bonding to the ribosomal machinery.

The 3' poly-A tail is recognized by PABP. RNA polymerase is involved in transcription, not translation. The 40s ribosomal subunit is recruited by the initiation complex (including eIF4e, PABP, and various other translation factors).

Heterochromatin often contains simple, repetitive sequences, and although it cannot be said that it is completely void of coding sequences, it is not typically transcribed. The tight wrapping prevents polymerase from accessing the strand, and euchromatin typically contains the regions that get transcribed. Thus, heterochromatin is though to contain repressed or inactive genes. 

The correct answer is guanine nucleotide exchange factors. In order to activate small GTPases and subsequently stimulate downstream pathways, guanine nucleotide exchange factors bind inactive GTPases and cause the release of guanine diphosphate (GDP). This allows guanine triphosphate (GTP) to bind and active the GTPase. 

Polyadenylation is an example of post-transcriptional modification. This process involves adding large repeats of adenine bases to the 3' end of mRNA molecules, known as the poly-A tail.

Myristoylation is the process of adding myristate (a fatty acid) to a protein, alkylation is the process of adding an alkyl group, and ubiquination is the process of adding a molecule of ubiquitin (a small protein often used to signal degradation). 

Two of the more common types of glycosylation, N-linked and O-linked, occur at different points and in different places in the cell. N-linked glycosylation takes place in the lumen of the endoplasmic reticulum, while O-linked glycosylation takes place in the Golgi body.

The other options, the mitochondria and the nucleus, are not involved in these post-translational modifications.

This question requires knowing either that CH₃CO is an acetyl group, or that acetylation is the most common protein modification. Each of the other modifications described are biologically occurring modifications, but acetylation was the correct answer for the given statement.