Option B: Some white X-brand cars with radial tyres are parked here.
     ● Statement 1: All X-brand cars parked here are white.  
       This implies that if a car is X-brand and parked here, it must be white.
     ● Statement 2: Some of them have radial tyres.  
       This indicates that among the X-brand cars parked here, there are some that have radial tyres.
     ● Statement 3: All X-brand cars manufactured after 1986 have radial tyres.  
       This suggests that if an X-brand car is manufactured after 1986, it will definitely have radial tyres.
     ● Statement 4: All cars are not X-brand.  
       This means that there are cars parked here that are not X-brand, but it does not negate the presence of X-brand cars.
     From these statements, we can conclude that there are some white X-brand cars with radial tyres parked here, as stated in Option B. This conclusion is supported by the combination of statements 1, 2, and 3.
     ● Option A: Only white cars are parked here.  
       This is incorrect because statement 4 indicates that not all cars are X-brand, and thus, not all cars are necessarily white.
     ● Option C: Cars other than X-brand cannot have radial tyres.  
       This is not supported by any of the statements. The statements do not provide information about the tyre types of non-X-brand cars.
     ● Option D: Most of the X-brand cars are manufactured before 1986.  
       This cannot be concluded from the given statements, as there is no information about the proportion of X-brand cars manufactured before or after 1986.
     Therefore, the most logical conclusion based on the given statements is Option B.