Through amniocentesis foetal cells can be cultures and tested for detecting various diseases if foetus by

The correct option is Karyotype.

Amniocentesis is a prenatal diagnostic procedure in which a small amount of amniotic fluid, which contains fetal tissues, is sampled from the amniotic sac surrounding a developing fetus. The fetal cells obtained from this fluid are then cultured in a laboratory.
Once these cells are cultured, the primary and most direct test performed on them is a karyotype analysis. Karyotyping involves staining the chromosomes of the dividing cells and observing them under a microscope to examine their number, size, and shape. This allows for the detection of chromosomal abnormalities such as Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), Patau syndrome (trisomy 13), and sex chromosome anomalies (such as Turner syndrome or Klinefelter syndrome).

While modern advanced techniques can analyze DNA or enzymes, the traditional and standard primary test directly associated with detecting fetal diseases via cultured cells from amniocentesis (especially in standard educational contexts) is checking the chromosomal constitution, which is known as Karyotyping. Therefore, based on the provided correct answer, karyotype is the designated method.