[a] It is the process by which heat is transmitted from one place to another without heating the intervening medium. Hence, it is not a reversible process.
[b] Nichrome wire is made of alloys and has high resistance. When current is passed through it heat is produced. So, here electrical energy is converted into heat energy. Hence, it is not a reversible process.
[c] It is the process by which heat is transmitted from one point to another through a substance in the direction of fall of temperature without the actual movement of the particles of the substances themselves. Hence, it cannot be reversible.
[d] Isothermal compression is reversible, for example-Cannot cycle. Heat engine. Thus, choice is correct.