The correct answer is C, $500,000 in equities. SIPC (Securities Investor Protection Corporation) provides protection for customers of failed broker-dealers up to $500,000 total per customer, including a maximum of $250,000 for cash.

Step-by-step, SIPC coverage applies to securities (stocks, bonds, mutual funds) and limited cash held in brokerage accounts. The key limits are:

Total coverage: $500,000

Cash sub-limit: $250,000

Now evaluate each choice:

A: $300,000 cash exceeds the $250,000 cash limit, so $50,000 would be uncovered.

B: $500,000 in cash exceeds the $250,000 limit, leaving $250,000 uncovered.

C: $500,000 in equities is fully covered because securities can be protected up to the full $500,000 limit.

D: Total is $750,000, which exceeds SIPC limits, so not fully covered.

Thus, only the account consisting entirely of $500,000 in securities (equities) falls within SIPC coverage limits without exceeding either the total or cash sub-limit.

Therefore, Answer C is correct.