The incorrect statement is (3) Trypsin is added to cell culture media to maintain cell’s health.

Option-wise explanation

(1) “Transformed cell lines do need external supply of serum to grow.” – Correct

Most mammalian cell lines, including transformed (immortalized or cancer) lines, are grown in media supplemented with serum (e.g., FBS) as a source of growth factors, hormones, and attachment factors. Even when serum‑free formulations are used, they are specially supplemented; routine transformed lines generally do require an external growth supplement.

(2) “The cells that are obtained directly from the organism is a primary culture.” – Correct

Cells freshly isolated from tissues and placed into culture for the first time are termed a primary culture. After subculture and adaptation they may become cell lines, but at the first isolation stage they are correctly called primary cultures.

(3) “Trypsin is added to cell culture media to maintain cell’s health.” – Incorrect

Trypsin is a protease used transiently to detach adherent cells from the culture vessel during passaging. It is added for a few minutes, then inactivated and removed by washing and adding fresh medium. Keeping trypsin in the growth medium would damage cell surface proteins and harm cell health, so it is not used as a routine medium component. This makes statement (3) the incorrect one.

(4) “HEPES buffer is generally used to maintain pH of the culture media.” – Largely correct

Many mammalian media rely mainly on bicarbonate/CO₂ buffering, but HEPES is often added as a supplemental buffer to stabilize pH when cultures are handled outside the CO₂ incubator. Thus, using HEPES to help maintain pH is a correct and common practice.

Therefore, the statement that is NOT correct for propagation and maintenance of mammalian cells in vitro is option (3).