**Core Concept**
Mycoplasmas are a group of bacteria that lack a cell wall, which is a critical component of the bacterial cell envelope. This characteristic renders them resistant to many antibiotics that target cell wall synthesis, such as beta-lactams. Mycoplasmas also possess unique metabolic and nutritional requirements, including the need for sterols, which are essential for their growth and survival.

**Why the Correct Answer is Right**
Mycoplasmas are not sensitive to beta-lactam group of drugs, such as penicillins and cephalosporins, because these antibiotics work by inhibiting cell wall synthesis, a process that is absent in Mycoplasmas. As a result, beta-lactam antibiotics are ineffective against Mycoplasma infections. **Option A** is incorrect because Mycoplasmas do multiply by binary fission, just like other bacteria. **Option C** is incorrect because Mycoplasmas can grow in cell-free media, but they require a specific set of nutrients and supplements to do so. **Option D** is incorrect because Mycoplasmas do require sterols for their growth, which is a unique characteristic of these bacteria.

**Clinical Pearl / High-Yield Fact**
Mycoplasmas are often difficult to culture and diagnose because they require specialized media and conditions to grow. However, they can cause a range of infections, including respiratory tract infections, genital infections, and conjunctivitis.

**✓ Correct Answer: B. Are sensitive to beta-lactam group of drugs**