The correct active form of the given sentence is:

☀ Thousands of people are expected to attend the memorial event.

So, the correct active form is: "Thousands of people are expected to attend the memorial event."

The other options and their explanations:

☀ Thousands of people was expected to attend the memorial event.

• This option uses the singular form "was expected" with the plural subject "thousands of people," which is not grammatically correct. The correct form is "were expected" if using the past tense.

☀ Thousands of people are expected to attending the memorial event.

• This option contains a grammatical error. The correct form should be "to attend," not "to attending."

☀ Thousands of people had expected to attending the memorial event.

• This option uses the past perfect tense ("had expected") and has an incorrect form of the verb "to attend" ("attending"). The correct form should be "had expected to attend." However, this option doesn't represent the active voice as requested.