For ligating a dephosphorylated vector what should the insert have?

Correct answer: phosphate group / Phosphate group

DNA ligase joins DNA fragments by forming a phosphodiester bond between a 3′-hydroxyl group and a 5′-phosphate group. When a vector is dephosphorylated, its 5′ phosphate groups have been removed. This prevents the vector ends from easily joining to each other and reduces self-ligation during cloning.

For the insert to be joined to the dephosphorylated vector, the insert must provide suitable 5′ phosphate groups. Ligase can then make the strand connection, although the resulting molecule may initially contain a nick that is repaired inside a host cell. Thus, the required feature is a phosphate group, so option A is correct; protein or ethanol cannot replace the DNA phosphate needed for ligation.