Sieve tubes differ from sieve cells in

The correct option is D Having sieve pores at end walls
Phloem is a vascular tissue. Its principal function is the conducting of assimilates and food. It is composed of the sieve elements, of which two types can be distinguished, sieve cells and sieve-tube members.Sieve cells are elongated cells with tapering end walls where as sieve tubes consist of vertical cells placed one above the other forming long tubes connected at the end walls by sieve pores.

So, the correct answer is 'Having sieve pores at end walls.'