Sieve-tube cells and companion cells work together to transport
sugar and nutrients inside plants. Together, sieve-tube cells and
companion cells form -
a. a tissue
b. an organ
c. an organ system
d. an organism

A) Tissue

Explanation:

Cells are the basic and fundamental units of all living organisms. Plants, like every other multicellular living organisms, are composed of cells that perform different functions. A group of cells that perform similar functions make up or form TISSUES, tissues that work together form the ORGAN, organs that work together to perform same function form the ORGAN SYSTEM.

In plants, sieve tubes and companion cells work together to carry out a similar task in plants, which is to transport nutrients and sugar inside the plant. Since sieve tubes and companion cells work together to fulfil same task, they collectively form a tissue in plants called PHLOEM tissue.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).