Estan Caminando isn't the answer as because "estan" implies to be another person, but this isn't the case as it's only the man, the man isn't walking (caminando either)
He isn't shopping (comprando) and he isn't painting (pintando)
The only logical answer is he's cooking (cocinando) which is also heavily implied as him having the bowl and whisk
B.) Esta Cocinando is your answer