For diatoms test, the best site for taking a sample is -
Lungs
Bone marrow in Ulna
Bone marrow in femur
Muscle
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