Final answer:
To increase the melting point of a polymer, you can either increase the length of the polymer chains or add a cross-linking substance. Hence, both option a and d are correct.
Step-by-step explanation:
Increasing the chain length of the polymer molecules generally leads to higher melting points, because longer chains have more points of contact and hence stronger van der Waals forces, known as London dispersion forces, which need more energy to break apart.
Cross-linked polymers have covalent bonds holding the chains together, creating a rigid three-dimensional network that is difficult to break down or melt. However, simply increasing the strength of C-C bonds within the polymer molecules does not necessarily alter the melting point.
Branching can lower the density and crystallinity of a polymer, making it less strong and easier to melt; thus, decreasing the number of short ramifications would likely increase the melting point by allowing better packing of the chains.