Step 1: Identify the tense of the sentence.
The phrase “was … at 6 o’clock yesterday morning” indicates an action that was in progress at a specific time in the past.
This corresponds to the Past Continuous Tense.
Step 2: Recall the structure of Past Continuous Tense.
\[
\text{was / were} + \text{verb + ing}
\]
Step 3: Apply the rule to the given sentence.
The verb given is “have”.
Its -ing form is:
\[
\text{have} \rightarrow \text{having}
\]
So the correct sentence becomes:
\[
\text{Mrs Adams was having dinner at 6 o’clock yesterday morning.}
\]
Step 4: Eliminate incorrect options.
(A) has — present tense, incorrect
(B) had — simple past, incorrect
(C) have — base form, incorrect
(D) having — correct \(-ing\) form
Hence, the correct answer is \(\boxed{\text{having}}\).