Hint: Nucleus is a spherical double membrane bound protoplasmic body which contains all the genetic information for controlling cellular metabolism and transmission of posterity.
Nucleus is an essential and integral part of the eukaryotic cell. It stores genetic information in its DNA molecules which can be passed on to progeny cells. Nucleus is considered as the 'control room' of a cell as it controls all cellular activities. The major cellular activities controlled by the nucleus are:
Nucleus contains hereditary material called chromatin which is a DNA-protein complex. Chromatin possesses all the genetic information that is required for growth and development of the organism, its reproduction, metabolism and behaviour.
Nucleus controls cell metabolism and other cellular activities through the formation of RNAs (mRNA, rRNA, tRNA) which control synthesis of particular types of enzymes.
Ribosomes are formed in the nucleolus part of the nucleus.
All variations of the organisms are caused by changes in genetic material present in the nucleus.
Nucleus also directs the synthesis of some structural proteins and chemicals required for cell growth and maintenance with the help of RNAs.
Replication of nucleus is mandatory for cell replication.
Therefore, having such major functions, a nucleus is also referred to as ‘the control room' of a cell.
Among the given options:
Mitochondria is called the powerhouse of cells because it is the major centre of release of energy in aerobic respiration.
Chloroplast is the centre of photosynthesis or formation of organic compounds from inorganic raw substances. It is known as ‘the kitchen of the plant cell’.
Ribosomes take part in protein synthesis, therefore they are popularly known as protein factories of the cell.
So, the correct answer is A.
Note: Nucleus acts as the control room of the cell and regulates almost all the activities of the cell. It instructs the cell to carry out basic functions like growth, development and division.