What is flex fuel engine in motor vehicles?

The correct option is: Engines in which biofuels and gasoline can be put in any ratio.

Understanding Flex Fuel Engines:
A Flexible Fuel Engine (commonly known as a flex-fuel engine) is an internal combustion engine designed to run on more than one type of fuel, or a mixture of fuels, in any proportion. Typically, these engines run on a blend of standard gasoline and a biofuel, such as ethanol or methanol.
Unlike traditional bi-fuel vehicles that store different fuels in separate tanks and switch between them (such as CNG and petrol), a flex-fuel vehicle stores the blended fuel in a single, common fuel tank.

How it Works:
Flex-fuel vehicles are equipped with a fuel composition sensor (or use engine sensor feedback) that automatically detects the exact ratio of ethanol/methanol to gasoline in the fuel line. The Engine Control Unit (ECU) then dynamically adjusts the fuel injection timing, injection quantity, and spark ignition timing to optimize combustion efficiency for whatever specific blend ratio is currently in the tank, whether it is 100% gasoline, 85% ethanol (E85), or any ratio in between.

Why the Other Options are Incorrect:
• Engines which run exclusively on biofuels: These are dedicated biofuel engines, not "flexible" fuel engines, because they lack the ability to run on standard gasoline.
• Engines which are compatible with only 50% biofuel and 50% gasoline: Flex-fuel engines are not restricted to a fixed 50:50 ratio; they can handle a wide variety of mixing ratios (commonly ranging from 0% biofuel up to 85% or 100% biofuel depending on the region and engine design).
• Engines which run on both diesel and CNG: These are typically classified as bi-fuel or dual-fuel systems, which keep the fuels in separate tanks, rather than flex-fuel systems where the fuels are mixed together in a single tank.