For diatoms test, the best site for taking a sample is -
Diatoms can be demonstrated in all the given options.

"The current technique for isolation of diatoms involve acid digestion of tissue-commonly bone marrow, lung, blood or kidneys with subsequent centrifugation and washing." - Parikh
"Bone marrow of long bone (e.g. femur) is examined for diatoms. Kidney, lung, liver or brain is also washed and 1 x1 cm, pieces from the deeper tissue." - Reddy

But, the examiner is specifically asking for ideal tissue, which is bone marrow of long bones:

­"The bone marrow is highly suitable and reliable. The bone marrow of long bones, such as femur, tibia, and humerus or sternum is examined for diatoms" - Reddy
Lungs
Bone marrow in Ulna
Bone marrow in femur
Muscle